Travis: move the valgrind run to allowed_failures

We don't need to know the result of the travis run before determining
whether we want to consider the merge as successful.
Travis: do non-threadsafe builds

When flipping the default to threadsafe, we never made sure that the
tests were building non-threadsafe variants. Specify which thead-safety
we want for each.
Merge pull request #2727 from libgit2/cmn/travis-jobs

Regain non-threadsafe builds and make the valgrind build failable