Force Vagrant-compatible Bundler version in Travis

Vagrant is picky about what version of Bundler it will work with. See:

- https://travis-ci.org/nsidc/vagrant-vsphere/builds/45393650
- https://github.com/mitchellh/vagrant/commit/3955d533f3c1994f243006a14359f1bba1b39e19
Add Travis-CI badge.