cannot create class "net.jackofalltrades.idea.action.CloseOtherProjectsAction"

In Intellij IDEA 13.0.1 on OS X. Plugin version 0.5. This exception comes right at the start of Intellij. Also:
$ java -version java version "1.6.0_65" Java(TM) SE Runtime Environment (build 1.6.0_65-b14-462-11M4609) Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-462, mixed mode)
cannot create class "net.jackofalltrades.idea.action.CloseOtherProjectsAction" [Plugin: net.jack-of-all-trades.idea.missing.functionality]: cannot create class "net.jackofalltrades.idea.action.CloseOtherProjectsAction" [Plugin: net.jack-of-all-trades.idea.missing.functionality]
com.intellij.diagnostic.PluginException: cannot create class "net.jackofalltrades.idea.action.CloseOtherProjectsAction" [Plugin: net.jack-of-all-trades.idea.missing.functionality]
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.a(ActionManagerImpl.java:280)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.a(ActionManagerImpl.java:232)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.getAction(ActionManagerImpl.java:225)
    at com.intellij.openapi.actionSystem.DefaultActionGroup.unStub(DefaultActionGroup.java:301)
    at com.intellij.openapi.actionSystem.DefaultActionGroup.getChildren(DefaultActionGroup.java:259)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl$5.compute(ActionManagerImpl.java:1201)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl$5.compute(ActionManagerImpl.java:1194)
    at com.intellij.openapi.application.impl.ApplicationImpl.runReadAction(ApplicationImpl.java:945)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.a(ActionManagerImpl.java:1194)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.a(ActionManagerImpl.java:1209)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.a(ActionManagerImpl.java:1209)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.preloadActionGroup(ActionManagerImpl.java:1188)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.c(ActionManagerImpl.java:1180)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.access$200(ActionManagerImpl.java:70)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl$4.run(ActionManagerImpl.java:1161)
    at com.intellij.openapi.application.impl.ApplicationImpl$8.run(ApplicationImpl.java:436)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
    at java.util.concurrent.FutureTask.run(FutureTask.java:138)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:695)
    at com.intellij.openapi.application.impl.ApplicationImpl$1$1.run(ApplicationImpl.java:153)
Caused by: com.intellij.diagnostic.PluginException: net/jackofalltrades/idea/action/CloseOtherProjectsAction : Unsupported major.minor version 51.0 [Plugin: net.jack-of-all-trades.idea.missing.functionality]
    at com.intellij.ide.plugins.cl.PluginClassLoader.b(PluginClassLoader.java:130)
    at com.intellij.ide.plugins.cl.PluginClassLoader.a(PluginClassLoader.java:77)
    at com.intellij.ide.plugins.cl.PluginClassLoader.loadClass(PluginClassLoader.java:66)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:249)
    at com.intellij.openapi.actionSystem.impl.ActionManagerImpl.a(ActionManagerImpl.java:255)
    ... 22 more
Caused by: java.lang.UnsupportedClassVersionError: net/jackofalltrades/idea/action/CloseOtherProjectsAction : Unsupported major.minor version 51.0
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:637)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:621)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:471)
    at com.intellij.util.lang.UrlClassLoader._defineClass(UrlClassLoader.java:189)
    at com.intellij.util.lang.UrlClassLoader.defineClass(UrlClassLoader.java:185)
    at com.intellij.util.lang.UrlClassLoader._findClass(UrlClassLoader.java:161)
    at com.intellij.ide.plugins.cl.PluginClassLoader.b(PluginClassLoader.java:124)
    ... 28 more
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