Downgrade rbenv-ruby-2.1.1 bundled rubygems version

Downgrade it to 2.1.11 (latest 2.1.x) to work around bundler issue with
rubygems 2.2.x (https://github.com/bundler/bundler/issues/2847)
Merge pull request #30 from alphagov/rbenv_ruby_211

Downgrade rbenv-ruby-2.1.1 bundled rubygems version