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.

**N.B. I was not able to test this against any bundler `~> 1.5.2` yet!  Please test that before merging.  It seems to work for me with 1.6.2 so far, though I have not done anything other than a basic `vagrant up`.**