Drop rubygems install for trusty / icehouse

ruby packaging in trusty includes ruby gem command already. Also
installing rubygems is unnecessary and results in multiple ruby
toolchains being installed.

Change-Id: Ic5bc93c0926c29fa5116f9cf7a0a3b23addda3ba
Closes-Bug: #1295327
Merge "Drop rubygems install for trusty / icehouse"