Don't run the validation if the PR's code can't be cleanly merged

[dotta]

Example, scala-ide/scala-ide#309 (comment)

I believe this is a regression, as this didn't use to happen in the past.

[adriaanm]

for the record, this depends on what your jenkins job does -- if it merges before running validation, you'll get a failure immediately when the merge fails and validation won't be run

[adriaanm]

(for some defintion of validation)