Temporarily for travis to use rubygems 2.1.x

Fixes https://tickets.opscode.com/browse/CHEF-4916
Merge branch 'rubygems-bundler-ruby18-travis-temp-fix'

Fixes https://tickets.opscode.com/browse/CHEF-4916

This should be reverted when Rubygems 2.2.1 becomes available with this patch:
https://github.com/rubygems/rubygems/commit/f8e0f1d5f67cfc4e1966cc1e2db367aebf8a09e4