Fix installation docs for new Vagrant versions

Old installation command fails on newer Vagrant versions, including 1.5.1 and 1.6.2.

```console
$ sw_vers
ProductName:	Mac OS X
ProductVersion:	10.9.2
BuildVersion:	13C1021
$ ruby -v
ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-darwin12.0]
$ gem -v
2.2.2
```
```console
$ vagrant -v
Vagrant 1.5.1
$ vagrant plugin install vagrant-ombnibus
Installing the 'vagrant-ombnibus' plugin. This can take a few minutes...
The plugin 'vagrant-ombnibus' could not be installed because it could not
be found. Please double check the name and try again.
```
```console
$ vagrant -v
Vagrant 1.6.2
$ vagrant plugin install vagrant-ombnibus
Installing the 'vagrant-ombnibus' plugin. This can take a few minutes...
The plugin 'vagrant-ombnibus' could not be installed because it could not
be found. Please double check the name and try again.
```