Tired of waiting for the Omnibus installer to download

An experimental pull request.

I got tried of waiting for the Omnibus installer to download on the provisionerless Bento boxes. To remedy that, this pull request:

- Replaces the default `install.sh` script with https://gist.github.com/hectcastro/6443633
- Ensures that `require_chef_omnibus` is set to a specific version of Chef and not a boolean
- Adds a `use_cachier_plugin` item via https://github.com/opscode/kitchen-vagrant/pull/37

Results __without__ this patch:

```bash
       Finished converging <default-ubuntu-1204> (1m20.29s).
-----> Setting up <default-ubuntu-1204>
       Finished setting up <default-ubuntu-1204> (0m0.00s).
-----> Verifying <default-ubuntu-1204>
       Finished verifying <default-ubuntu-1204> (0m0.00s).
-----> Destroying <default-ubuntu-1204>
       [kitchen::driver::vagrant command] BEGIN (vagrant destroy -f)
       [default] Forcing shutdown of VM...
       [default] Destroying VM and associated drives...
       [kitchen::driver::vagrant command] END (0m6.58s)
       Vagrant instance <default-ubuntu-1204> destroyed.
       Finished destroying <default-ubuntu-1204> (0m7.16s).
       Finished testing <default-ubuntu-1204> (2m14.59s).
-----> Kitchen is finished. (2m15.02s)
```

Results __with__ this patch:

```bash
       Finished converging <default-ubuntu-1204> (0m57.85s).
-----> Setting up <default-ubuntu-1204>
       Finished setting up <default-ubuntu-1204> (0m0.00s).
-----> Verifying <default-ubuntu-1204>
       Finished verifying <default-ubuntu-1204> (0m0.00s).
-----> Destroying <default-ubuntu-1204>
       [kitchen::driver::vagrant command] BEGIN (vagrant destroy -f)
       [default] Forcing shutdown of VM...
       [default] Destroying VM and associated drives...
       [kitchen::driver::vagrant command] END (0m6.16s)
       Vagrant instance <default-ubuntu-1204> destroyed.
       Finished destroying <default-ubuntu-1204> (0m6.72s).
       Finished testing <default-ubuntu-1204> (1m54.40s).
-----> Kitchen is finished. (1m54.85s)
```