Add Vagrant version constraints and checks (Fixes #26)

As per #26 I've created an initial stab at checking the versions. I haven't tested this yet. @fgrehm can you pull and test this?

#### Part of the discussion on #26
@fgrehm
> I suspect this change might break things over here
> 
> /cc @patcon @ipwnstuff

@mitchellh
> Since bindler is so dependent on some internal Vagrant state, I recommend putting this sort of check in the plugin.rb file

@ipwnstuff
> I think it's handled well in vagrant-berkshelf see [the install action they've defined](https://github.com/berkshelf/vagrant-berkshelf/blob/master/lib/berkshelf/vagrant/action/install.rb#L40-L71). If you agree I'm up for creating a PR.