Ignore gem files in project root
Add vagrant-spec to test dependencies

`vagrant-spec` is a library of RSpec contexts and helpers that makes it easier
to test Vagrant plugins.
Add acceptance tests

Starting with a simple acceptance test that boots a CentOS 6.5 machine and
installs a PE agent 3.2.3 on it.
Move version validation into provisioner

By validating a PE version was set after the provisioner merges configs, we can
ditch a bunch of troublesome logic in `Config::PEBootstrap` that messed with
global state.