Add CMakeCache.txt to .gitignore.

Change-Id: I165a3e2f17a50f9f21456a1690862db38b483c7e
Reviewed-on: http://review.couchbase.org/39816
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>