update .gitignore

Change-Id: Ia5102ce60a5db46604908ecbc74c0b964a8a9b35
Reviewed-on: http://review.couchbase.org/29676
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>