travis: use an environment variable for the gitter webhook
travis: use an environment variable for the gitter webhook

This makes sure forks of machinekit/machinekit will not use the same
gitter webhook.

ArcEye/MK-Qt5,kinsamanka/machinekit,mhaberler/machinekit,araisrobo/machinekit,mhaberler/machinekit,mhaberler/machinekit,mhaberler/machinekit,araisrobo/machinekit,ArcEye/MK-Qt5,mhaberler/machinekit,araisrobo/machinekit,kinsamanka/machinekit,strahlex/machinekit,ArcEye/MK-Qt5,kinsamanka/machinekit,kinsamanka/machinekit,ArcEye/MK-Qt5,kinsamanka/machinekit,ArcEye/MK-Qt5,strahlex/machinekit,araisrobo/machinekit,strahlex/machinekit,mhaberler/machinekit,strahlex/machinekit,kinsamanka/machinekit,araisrobo/machinekit,ArcEye/MK-Qt5,kinsamanka/machinekit,strahlex/machinekit,araisrobo/machinekit,ArcEye/MK-Qt5,ArcEye/MK-Qt5,araisrobo/machinekit,strahlex/machinekit,mhaberler/machinekit,araisrobo/machinekit,kinsamanka/machinekit,strahlex/machinekit,araisrobo/machinekit,mhaberler/machinekit