Update snappy import path.

It has recently changed in https://github.com/golang/snappy/pull/14#issuecomment-122197952.
diff --git a/database/database.go b/database/database.go
index ceb034c..c893826 100644
--- a/database/database.go
+++ b/database/database.go
@@ -48,7 +48,7 @@
 	"github.com/garyburd/redigo/redis"
 	"github.com/golang/gddo/doc"
 	"github.com/golang/gddo/gosrc"
-	"github.com/golang/snappy/snappy"
+	"github.com/golang/snappy"
 )
 
 type Database struct {