Update rvm version to `stable`

I don't know if you guys purposely wanted to keep an old version of rvm because of compatibility reasons, but I was running 1.19.1, and wanted to update to the latest, and so I updated the version to `stable`, which as of this writing is 1.19.6.

If you guys don't want to take this update, let me know, and just keep a separate copy in my fork. Thanks.

### Changes
* update pivotal_workstation/attributes/versions.rb and set rvm version to stable
* add pivotal_workstation/changelog.md
* up the pivotal_workstation version in meta data to 1.0.1.pre