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