Awesome way of checking Vagrant version!

Makes it a lot easier to write specs for too. Totally stolen from @tmatilai in his excellent https://github.com/tmatilai/vagrant-proxyconf code :+1:
Adding error for Vagrant version mismatch
More plugin specs

This gives us `95.24%` coverage on this file! :+1:
Move require_relative back to action_hook

This is so it's supposed to be lazy loaded (See https://docs.vagrantup.com/v2/plugins/development-basics.html for details)
Fixing plugin spec with mock for constant
Fixes travis issue with Vagrant bundler version...

See https://github.com/mitchellh/vagrant/issues/3451 for more details! :+1:
Rubocop fixes
Add line-length exception
Fix for Rubucop:

"Avoid using rescue in its modifier form."
Rubcop fix: 

Favor unless (or control flow or) over if for negative conditions.
Locking down versions

Travis issues with Rspec > 3 incompatibilities.
Merge pull request #92 from petems/add_plugin_spec

Add plugin spec and improve Vagrant version checking