- nm-ovsdb.c uses json_load_callback(), which is jansson v2.4.
  Hence, it cannot build the OVS plugin in our Travis-CI, which is
  still on Ubuntu Precise. Disable building the plugin in travis and
  add a compiler warning when building against an older version.

- since jansson v2.3, there is json_object_key_to_iter() to implement
  the for-each macros. Use it in json_object_foreach_safe() when
  available.