travis: only test riak with python 2.7, do not use deprecated pip --use-mirror option
travis: only test riak with python 2.7, do not use deprecated pip --use-mirror option

olt/mapproxy,olt/mapproxy,mapproxy/mapproxy,geoadmin/mapproxy,camptocamp/mapproxy,procrastinatio/mapproxy,drnextgis/mapproxy,vrsource/mapproxy,vrsource/mapproxy,drnextgis/mapproxy,geoadmin/mapproxy,procrastinatio/mapproxy,camptocamp/mapproxy,mapproxy/mapproxy