[scikit-learn] Disable Travis Cache

[Raghav R V]

Could someone disable the Travis cache once and for all please?

I have seen several frustrating incidents where the Travis fails the PR
because of this caching of old files.

I also don't understand why it is enabled in the first place. It would
really be super helpful if it is disabled for good.

Also refer - https://github.com/scikit-learn/scikit-learn/issues/7094

**cc**: Olivier, Andy
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