Revert "Temporarily disable IDE PR validation"

This reverts commit cf5cb7502d1708d238efadcb9cfecafe112cc76a.

I accidentally disabled partest integration, rather than IDE
validation.
Disable IDE valiation on the 2.12.x branch.

Pending upstream build work in Scala IDE.
Merge pull request #107 from retronym/topic/ide-2

Disable IDE valiation on 2.12