[buildbucket] Fix printing build url

TBR=smut@chromium.org,nodir@chromium.org

Bug: 711249
Change-Id: I031497eeeec02f16c182fbe2e46c6a02f0745df9
Reviewed-on: https://chromium-review.googlesource.com/1232896Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>

buildbucket.py

@@ -134,7 +134,7 @@ def main(argv):
   if args.verbose:
     print content
 
-  build_url = content.get('build', {}).get('url')
+  build_url = json.loads(content).get('build', {}).get('url')
   if build_url:
     print 'Build triggered on: %s' % build_url