Don't use obsoleted URI.regexp

`URI.regexp` is obsoleted since Ruby 2.2.

```
$ ruby -w -r "uri" -e "URI::regexp"
-e:1: warning: URI.regexp is obsolete
```

lib/openid_connect/client/registrar.rb

@@ -118,7 +118,7 @@ def sector_identifier_required?
 
       def valid_uri?(uri, schemes = ['http', 'https'])
         # NOTE: specify nil for schemes to allow any schemes
-        URI::regexp(schemes).match(uri).present?
+        URI::DEFAULT_PARSER.make_regexp(schemes).match(uri).present?
       end
 
       def validate_contacts