Allowing user to set a default JRuby version

This provides the ability to set a global jruby version and (for now) allow ```jrubyWar``` to utilise it. ```JRubyPlugin``` is extended to use ```project.afterEvaluate``` and ```JRubyExtension``` now has a ```defaultVersion``` added.

Enhancements that can follow out of this:
+ Handle ```jruby-rack``` in a similar way
+ Easier was for JRubyExec (when it gets written) to have a global default.