Install bundler on jenkins slaves

This commit adds bundler to the list of gems that need to
be installed on jenkins slave.

Bundler is the ruby equivalent of virtualenv, it allows tests to
install their own sets of gems into the local test directory
without attempting to install them as system gems. This is highly
recommended for any ruby unit tests that have their own sets of
gem dependencies (specifically, the puppet modules which will
be joining stackforge).

Change-Id: Ic443e6bf751cbf1604e4ab6b8910f45abb7eb8dd
Reviewed-on: https://review.openstack.org/26053
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins