Update fugue 2.x to be guava 23.x compatible

[Richard Atkins]

Legacy code using fugue 2.x APIs cannot migrate to the latest version of guava, due to problems in Option and Iterables. We need to upgrade this legacy release of fugue to be guava 23.x compatible, so we can move to the new platform.

The relevant change in the latest guava versions is:

• http://google.github.io/guava/releases/20.0/api/diffs/changes/com.google.common.collect.Iterators.html com.google.common.collect.Iterators.emptyIterator removed. Use Collections.emptyIterator instead.

Affected classes are: * com.atlassian.fugue.Option * com.atlassian.fugue.Iterables

[Richard Atkins]

Making legacy fugue compatible with guava 23.1-jre requires updating the java version to 1.7+. All current products that use fugue are already running with java 1.8, so we can move to a base pom version that requires that too. That change makes this bigger than a simple bugfix, so I'll bump the library version to 2.7.0 as an indicator that its dependencies have changed - 3.x already shipped with the io.atlassian.fugue packaging change, which is also source and binary incompatible with the 2.x series

[Alex Wei]

Can this be changed to "resolved"?

[Anund McKague]

Fixed in https://bitbucket.org/atlassian/fugue/pull-requests/87