Update .gitignore rules to exclude gtest artifacts

Change-Id: I33e123b118a2bc02a20e515b035a38f8c447d55b
Reviewed-on: http://review.couchbase.org/28872
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>