Switch ruby jobs to ruby 2.0

* Migrate all browser tests to use ruby2.0/gem2.0 on Trusty instances.

* Namespace GEM_HOME based on ruby version

A job migrated to Trusty/ruby2.0 ended up failling with a segfault in
the json gemfile. The gem uses some compiled code which is hold in a per
instance cache by setting GEM_HOME.  Since that cache is populated via
ruby 1.9.x, the compiled version is not going to work with ruby 2.x

Namespace the per instance cache (GEM_HOME) by using the ruby version as
reported by RUBY_VERSION ie '2.0.0'.

Side effect: the ruby 1.9.x based cache is no more being used, hence
jobs will repopulate it as jobs are being build.

Change-Id: Ibc06e59966c37dbfdda6f30a03f2e12b36eb60e4