Make mixins.py use the in-repo treeherder-client

The revert of bug 1080760 added back an import of the treeherder-client
package, which escaped conversion since it was not present when the
client was imported to the repo.

treeherder/etl/mixins.py

@@ -10,7 +10,7 @@
 
 import simplejson as json
 
-from thclient import TreeherderRequest
+from treeherder.client import TreeherderRequest
 
 from django.core.urlresolvers import reverse
 from django.conf import settings