Extract `Plugin.check_vagrant_version` helper
Adapt plugin loading for Vagrant 1.5

`Vagrant.require_plugin` will be deprecated in Vagrant 1.5.

In new Vagrant versions we now use `require` directly for loading the
optional dependencies. All exceptions are logged but ignored. Vagrant
itself will handle the real errors later (or has already errored out).
Merge pull request #44 from tmatilai/require_plugin_deprecated

`Vagrant.require_plugin` will be deprecated in Vagrant 1.5