Increase the time we wait for a registry from 0.5 to 5 seconds

We are not testing registry start-up performance, and killing the test
suite just because Travis is a bit busy doesn’t help; we’re much better
off with a test run which gives the registry a bit more time.

integration/registry.go

@@ -31,7 +31,7 @@ func setupRegistryV2At(c *check.C, url string, auth, schema1 bool) *testRegistry
 	c.Assert(err, check.IsNil)
 
 	// Wait for registry to be ready to serve requests.
-	for i := 0; i != 5; i++ {
+	for i := 0; i != 50; i++ {
 		if err = reg.Ping(); err == nil {
 			break
 		}