Temporary workaround for failing 1.8.7 Ruby/Travis

Rubygems broke Travis/Bundler builds on Ruby 1.8.7:

* travis-ci/travis-ci#1793
* rubygems/rubygems#763

This patch is a temporary workaround until rubygems is updated.
Merge pull request #74 from hogepodge/bundler_workaround

Temporary workaround for failing 1.8.7 Ruby/Travis