Temporary fix for rb 1.8.7 on Travis CI

Ruby 1.8.7 is broken otherwise, but doing this fixes it. No harm in running it for other rubies either.

See https://github.com/bundler/bundler/issues/2784 and https://github.com/travis-ci/travis-ci/issues/1793 for more context on the bug.

We should revert this once Travis has fixed it and rolled the fix out.