Adding merge_commit_sha field to PullRequest model

[jdesiloniz]

This PR adds the merge_commit_sha to the PullRequest model. According to the GitHub API docs:

The value of the merge_commit_sha attribute changes depending on the state of the pull request. Before merging a pull request, the merge_commit_sha attribute holds the SHA of the test merge commit. After merging a pull request, the merge_commit_sha attribute changes depending on how you merged the pull request:

If merged as a merge commit, merge_commit_sha represents the SHA of the merge commit.
If merged via a squash, merge_commit_sha represents the SHA of the squashed commit on the base branch.
If rebased, merge_commit_sha represents the commit that the base branch was updated to.

This field can be really useful for creating and updating statuses by targetting the test commit sha instead of the latest one in a PR.

[fedefernandez]

@jdesiloniz I guess we need to disable that option for scala 2.11, something like:

scalacOptions ++= (scalaBinaryVersion.value match {
  case x if x.startsWith("2.12") => Seq("-Ywarn-macros:after")
  case _                         => Seq.empty
})

[bilki]

This LGTM, aside from the compilation problems (that seem to be solved?)

[bilki]

Is this still necessary?

[jdesiloniz]

@bilki I've just launched a new build process in Travis without these, to verify if the stack size fix did the trick by itself... 🤔

[jdesiloniz]

@bilki yup, that did the trick! Thanks for the suggestion!!

[bilki]

Are we going to release now?

[jdesiloniz]

@bilki I'm holding back from this, so we can decide the release timeline appropiatedly in a separate PR.

[fedefernandez]

retroactive approval, looks good to me