update travis config to use debian python and install mock
update travis config to use debian python and install mock

ros/catkin,shyamalschandra/catkin,shyamalschandra/catkin,shyamalschandra/catkin,wkentaro/catkin,scpeters/catkin,asmodehn/catkin,beni55/catkin,asmodehn/catkin,asmodehn/catkin,beni55/catkin,scpeters/catkin,harmishhk/catkin,thaddeustaylor/catkin,harmishhk/catkin,ros/catkin,beni55/catkin,thaddeustaylor/catkin,thaddeustaylor/catkin,wkentaro/catkin,wkentaro/catkin,ros/catkin,scpeters/catkin,jack-oquin/catkin,jack-oquin/catkin,harmishhk/catkin,jack-oquin/catkin