Rename gem to conform to vagrant plugin naming convention

Not sure this ticket is worthwhile but I did just noticed the name of the Berkshelf Vagrant seems to be different then seemingly established naming conventions for most other plugins.  While I do agree that Berkshelf is better then all other plugins, and as such maybe it needs to differentiate itself, I still thought a ticket might be worthwhile in case you guys felt otherwise  :P

```shell
jdyer@dyer:~/Dropbox/Projects/deployment_models/phono-gateway-model/gateway_server(master⚡) » vagrant plugin list                                      1 ↵
berkshelf-vagrant (1.1.0)
thor (0.18.1)
vagrant-aws (0.1.3)
vagrant-awsinfo (0.0.4)
vagrant-omnibus (1.0.0)
vagrant-vbguest (0.7.1)
vagrant-vmware-fusion (0.4.2)
```