Merged two job templates that build Ruby gems into one

Tested building a gem on my machine and it works fine:

$ gem build *.gemspec
Successfully built RubyGem
Name: mediawiki_selenium
Version: 0.2.15
File: mediawiki_selenium-0.2.15.gem

Bug: 58040
Change-Id: I6c6db23fc830d951f61da8eca6639b4e96101c57