Compilers on e44

A fix to Groovy compiler resolver plugin! Hence, a quick code review would be nice :-)

Fix for Groovy Compilers mix up on Groovy compiler preferences page. Can only be reproduced with Greclipse on e44!

The code in CompilerChooser assumes that if compiler version is not specified than it's the groovy version bundled in the latest version of org.codehaus.groovy plugin available.
Turns out that with e44 M6 this assumption doesn't hold anymore. The plugin loaded all the time is the first installed one. Namely the org.codehaus.groovy 2.1 plugin with Groovy 2.1.8 in it. Consecutive installs of Groovy 2.2 and 2.3 features don't result in the latest org.codehaus.groovy plugin activated.
Looks like the reason behind org.codehaus.groovy version 2.1 being loaded and not the latest is some Eclipse/OSGI magic or defect in Luna M6. However, we should be able to still display the correct version.
Built Greclipse update-site locally for e44 and e43j8
Tested the fix with GGTS 3.5.1 on e44
Tested the fix with GGTS 3.5.1 on e43j8. Verified that behavior is still the same in e43 and the fix doesn't break it.