Remove Rakefile installation

This functionality is supplanted by the CLI/Installer.

Intentionally removing Causes-specific repo detection; I may add this
back as an option, but it doesn't really fit anywhere in a
publicly-released gem.

Change-Id: I3c4a0232473acaab21f81c86aaaf5f5f61592406
Reviewed-on: https://gerrit.causes.com/22700
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>