Added info about Vagrant 1.5 Compatibility

To avoid issues about the plugin not working on 1.5
Merge pull request #157 from Grafikart/patch-1

Added info about Vagrant 1.5 Compatibility
Fix for Vagrant 1.5

- Drop support for Vagrant < 1.5
- Drop test for Ruby 1.9.3
- Remove usage of config_global
- Fix UI initialization
Specify Vagrant 1.5.x