travis: run valgrind only once

Instead of running valgrind on each job, half of which are in release
mode and don't have much usable information for valgrind, perform an
debug build as part of allowed_failures and run valgrind on that one,
which should speed up the feedback we get from the builds.
Merge pull request #2569 from libgit2/cmn/valgrind-once

travis: run valgrind only once