code.google.com is shutting down so update the dep

Signed-off-by: Jessica Frazelle <acidburn@docker.com>

graphdb/conn_sqlite3_unix.go

@@ -3,5 +3,5 @@
 package graphdb
 
 import (
-	_ "code.google.com/p/gosqlite/sqlite3" // registers sqlite
+	_ "github.com/mattn/go-sqlite3" // registers sqlite
 )

graphdb/graphdb_test.go

@@ -8,7 +8,7 @@ import (
 	"strconv"
 	"testing"
 
-	_ "code.google.com/p/gosqlite/sqlite3"
+	_ "github.com/mattn/go-sqlite3"
 )
 
 func newTestDb(t *testing.T) (*Database, string) {