CMake: Use gtest 1.7.0

Change-Id: I60484da7e131ef41a569a68e72a5a2417adf328a
Reviewed-on: http://review.couchbase.org/31735
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>