Work-around stale TRAVIS_COMMIT variable
Work-around stale TRAVIS_COMMIT variable

There seem to be occasions when rebuilding PRs where the TRAVIS_COMMIT
variable (which should be the SHA of the merge commit) is out-of-date.
When this happens, and the commit referred to does not exist, update
TRAVIS_COMMIT to be FETCH_HEAD as retrieved from GitHub instead.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml