Update OpenShift Docs for Ruby 2.0 Cartridge 

Small update to the OpenShift docs to use OpenShift's latest Ruby 2.0 Cartridge. But OpenShift's Bundler still seems a little old, resulting in some workarounds.
You could collapse steps 5 and 6 by using the following command for the current step 6.
```sh
rhc env set SECRET_TOKEN="`openssl rand -hex 20`"
```
See the [VPS docs](https://github.com/jasonngpt/stringer/blob/63ac30145b035a54c82049185f4060cc8cc789d4/docs/VPS.md#install-stringer-and-set-it-up).