Allowing specifying rake version from the environment

Puppet 5's rake version is pinned to a fairly old version for Ruby 1.9 compatibility. This causes issues with the packaging as a gem work, requiring that packaging in Puppet 5 be pinned to older versions as well.

Since we only need this old Rake in Travis for testing against old Ruby's, we should only pin it in that case (and possibly in the final Gem we build), but not generally. That way packaging can be loaded in our internal build/test processes.