Adapt to mapquest discontinuing free service

As of July 11, MapQuest has disabled their free map tile feed. This means that apps using CHAMPLAIN_MAP_SOURCE_OSM_MAPQUEST no longer display anything except a warning pointing to

http://devblog.mapquest.com/2016/06/15/modernization-of-mapquest-results-in-changes-to-open-tile-access/

Screenshot of the GNOME Maps app attached.

This was originally reported as bug 1602035 but that bug report is kind of full and the solution for gnome-maps (Mapbox donated one year of service) may not be transferable to other apps.

Here's a list of possibly affected apps:

$ reverse-depends src:libchamplain

* empathy (for libchamplain-0.12-0)
* eog-plugins (for libchamplain-0.12-0)
* fotoxx (for libchamplain-0.12-0)
* gnome-contacts (for libchamplain-0.12-0)
* libabiword-3.0 (for libchamplain-0.12-0)
* libevolution (for libchamplain-0.12-0)
* openstreetmap-client (for gir1.2-champlain-0.12)
* qreator (for gir1.2-champlain-0.12)