Fix development dependencies and Travis tests

vagrant-rackspace v0.1.3 has a conflicting fog dependency with vagrant-aws v0.3.0, so lock it down to 0.1.2 for now. This should fix the [Travis build](https://travis-ci.org/schisamo/vagrant-omnibus/builds/11586774).

Also skip installing the provided plugins at all in Travis as they are only used in acceptance tests.