Add a missing (string) cast to repository.query

Summary: The method returns a PhutilURI object, which json_encode() encodes as "{}" since all the properties are private.

Test Plan:
Examined the output of "repository.query" more carefully:

  "0" : {
    "name"      : "Phabricator",
    "callsign"  : "P",
    "vcs"       : "git",
    "uri"       : "http:\/\/local.aphront.com:8080\/diffusion\/P\/",
    "remoteURI" : "git:\/\/github.com\/facebook\/phabricator.git",
    "tracking"  : true
  },

Reviewers: csilvers, btrahan, vrana, jungejason

Reviewed By: csilvers

CC: aran

Differential Revision: https://secure.phabricator.com/D2402

src/applications/conduit/method/repository/base/ConduitAPI_repository_Method.php

@@ -27,7 +27,7 @@ abstract class ConduitAPI_repository_Method extends ConduitAPIMethod {
       'callsign'    => $repository->getCallsign(),
       'vcs'         => $repository->getVersionControlSystem(),
       'uri'         => PhabricatorEnv::getProductionURI($repository->getURI()),
-      'remoteURI'   => $repository->getPublicRemoteURI(),
+      'remoteURI'   => (string)$repository->getPublicRemoteURI(),
       'tracking'    => $repository->getDetail('tracking-enabled'),
     );
   }