test_fish.py: use official python images, not ubuntu
test_fish.py: use official python images, not ubuntu

This should help reduce build times.

mlk/thefuck,Clpsplug/thefuck,scorphus/thefuck,scorphus/thefuck,mlk/thefuck,nvbn/thefuck,Clpsplug/thefuck,SimenB/thefuck,nvbn/thefuck,SimenB/thefuck