Fixed source compatibility for latest master

Small fix due to Scala IDE PR #516
(https://github.com/scala-ide/scala-ide/pull/516)
Fixed source compatibility with Scala IDE master

Source compatibility with master was broken by this recent commit
scalaide/scalaide@cf73ce8a4c01c97a476f13e6af5f2237895e30dc
Added 'override' keyword where missing
Merge pull request #157 from dotta/compatibility/ide-pr-516

Fixed source compatibility for latest master
Merge pull request #162 from dotta/wip/update-codebase-to-ide-master

Wip/update codebase to ide master
Fixed compilation issue

This was a leftover of SHA:58744e4bd579ef7b428562e810174bebe889211e
Move to Scala OSGi bundles.
Bump plugin profile version.
Bump OSGi Execution environment to 1.6, to satisfy Scala compiler dependency on 1.6