Cache pip: install 60 -> 9 seconds, total 1m58s -> 1m12s

[hugovk]

No cache enabled:

Ran for 1 min 58 sec Total time 3 min 43 sec

59.76s pip install -r requirements.txt

https://travis-ci.org/hugovk/diffengine/jobs/210132306#L134

Cache enabled, no cache created yet:

Ran for 1 min 48 sec Total time 3 min 29 sec

49.62s pip install -r requirements.txt

Cache enabled, using cache:

Ran for 1 min 12 sec Total time 2 min 10 sec

9.07s pip install -r requirements.txt

https://travis-ci.org/hugovk/diffengine/jobs/210143455#L147