Revert "Ensure Travis uses the latest version of `xctool`."

This reverts commit 38fdea9b46bdd12b6cc7b45e1a14ab49b0c17c67,
which workaround is no longer necessary now that Travis has xctool 0.1.16 installed
(https://github.com/travis-ci/travis-ci/issues/2345).
Merge pull request #235 from inkling/jeff/fix_build

Update Subliminal's CI configuration for recent updates to Travis.