ign-gazebo6: use nightlies to test sdf12 issue

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

Author: scpeters

plugins/config/repository.yaml

@@ -49,6 +49,12 @@ projects:
             type: stable
           - name: osrf
             type: nightly
+    - name: ignition-gazebo6
+      repositories:
+          - name: osrf
+            type: stable
+          - name: osrf
+            type: nightly
     - name: ignition-gazebo7
       repositories:
           - name: osrf