GitLab

After losing jenkins, it's been difficult to test all commits
manually. This commit adds a Makefile target that makes travis do it.
Unfortunately, this is way too slow. It takes longer than the
allotted 10min.

After asking the travis people what to do, someone suggested making
sure that each commit is pushed to github independently. This makes
travis run CI on every single commit in the PR, and gives us nice
status indicators on each one (so we know which ones did not pass).

This approach means that we need to push a branch to the repo for
each commit in the PR-- otherwise travis may cancel its run if it
detects that the branch is no longer there. We could automate this
with a bot that essentially does:

  for each PR:
    git fetch the PR branch
    push a branch per commit: <branch>-<commit>

  for each closed PR:
    delete all branches with pattern <branch>-<commit>