[scikit-learn] Disable Travis Cache

[Gael Varoquaux]

On Mon, Aug 08, 2016 at 03:10:26PM +0200, Raghav R V wrote:
> I felt we could rather have a clean build and wait for a few more minutes (if
> that's the disadvantage of disabling caching) than have it pass / fail on old
> code...

Time of CI is a real problem. On our side because it slows down merges of
PRs, and for the infrastructure: it's server time and money for them.

I'd much rather have a working cache.

Gaël