add support for bundler >= 1.6.x

bundler changed their API for Bundler::UI in 1.6.0 via this commit:

  https://github.com/bundler/bundler/commit/5e38e2d7

This caused various issues, e.g.

  https://github.com/mitchellh/vagrant/issues/3451
  https://github.com/test-kitchen/test-kitchen/issues/401

which will hopefully be resolved by teaching vagrant to cope with both
API versions.
Merge pull request #3587 from aspiers/bundler-1.6

add support for bundler >= 1.6.x