Ensure build checks for JRuby

[Brett Giles]

Although Rake can be run even if there is a default ruby install, JRuby is required as it bundles an ant task that is used for compiling the Java and creating the jar file.

We need to check the version and type of "ruby" running when rake is invoked and give a helpful message when it is not JRuby of at least version 1.6.7

[Brett Giles]

Added:

if not (defined? RUBY_ENGINE && RUBY_ENGINE == 'jruby')
  abort 'Sorry - Building LQPL requires JRuby. You appear to be running or defaulted to some other ruby engine.'
end

and tested this with system ruby on mac. Seems to work as desired.