Bump bundler to 1.6.5

The current version of bundler is over a year out of date and the dependency resolution in newer versions is much better. Specifically, I'm having issues with the multi-ruby build version of gds-sso: https://github.com/alphagov/gds-sso/compare/multi-rails. Newer version of bundler correctly resolve the dependencies.

The main puppet repo has a separate pull request to bump the version on the rest of our infrastructure to match the version here.