cache pip for faster CI build

[lgarithm]

Currently pip install takes about 90s out of 3m30s per build.

not

[DEKHTIARJonathan]

Does it actually works? I tried to do it but never succeeded 😅

[lgarithm]

Not yet, maybe some additional steps are required:
https://docs.travis-ci.com/user/caching/#before_cache-phase
Or maybe this needs to be on master to make it work for later CIs.

[lgarithm]

Basically I'd like to cache the .whl files downloaded by pip.
I need to make sure they are stored in $HOME/.cache/pip.

[lgarithm]

@luomai @DEKHTIARJonathan I think the cache is working now.
pip install is using cache and the time it takes reduced to 45s from 90s.

[DEKHTIARJonathan]

Fine with me I merge 😊