Fix: Ensure travis uses rubygems 2.1.11 for 1.8.7

When using ruby 1.8.7 and rubygems 2.2.0 - running `bundle install`
fails with the following error:

$ bundle install
Fetching gem metadata from http://rubygems.org/.........
Fetching additional metadata from http://rubygems.org/..
Unfortunately, a fatal error has occurred. Please see the Bundler
troubleshooting documentation at http://bit.ly/bundler-issues. Thanks!
/1.8/rubygems/specification.rb:325:in `sort':
  undefined method `<=>' for nil:NilClass (NoMethodError)
/1.8/rubygems/specification.rb:325:in `files'
/1.8/rubygems/specification.rb:2155:in `send'
/1.8/rubygems/specification.rb:2155:in `same_attributes?'
/1.8/rubygems/core_ext/kernel_require.rb:55:in `all?'