Get version string from scm

Change-Id: Ibea935d6a6592145b12b97ed43c21507b687bc4a
Reviewed-on: http://review.couchbase.org/32719
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>