bundle check before bundle install for gem tests

they're likely already installed, and it will save a ton of trips
to rubygems.org

Change-Id: I9ccf2194619a6e8f97d7f21b4e232dac7ff20d3c
Reviewed-on: https://gerrit.instructure.com/35694
Reviewed-by: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>