Make Vagrant version configurable for testing

`VAGRANT_VERSION` environment variable can be used for specifying the
wanted Vagrant version. Defaults now to v1.4.2.
travis: Test against master all supported minor Vagrant versions

Use correct Ruby version with each Vagrant version.

Allow the master build to fail. (And it will now as
`Vagrant.require_plugin` is deprecated.)
Merge branch 'test-multiple-vagrant-versions'