Force single-job bundle install

This avoids problems with corrupt gem files.

This override of standard Travis CI configuration will probably no
longer be needed with Rubygems 2.5. See also
https://github.com/rubygems/rubygems/issues/982.
Merge pull request #1358 from mvz/fix-travis

Force single-job bundle install
Don't allow failures on Rubinius
Revert "Don't allow failures on Rubinius"

This reverts commit 70da152cf5ecd81e95f06df3e6c0697582d39075.