CBLT-332: compute sync_gateway S3 path from SYNCGATE_VERSION

Sometimes android source is branched before sync_gateway, so until
the sync_gateway builds can set the SYNCGATE_VERSION we may have
to use an old one.

Change-Id: I922b68e05ac07d46a0e37423d20861278923fdab
Reviewed-on: http://review.couchbase.org/42003
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>