Ignore generated VERSION file

Change-Id: Id727e57d619c101a5dd4dffe900a3b61d806aab9
Reviewed-on: http://review.couchbase.org/27955
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>