Try conditional caching of miniconda

[adelavega]

Trying to fix random miniconda failures, inspired by: https://github.com/theochem/cgrid/blob/03a995a602fe043ee9c05e3797ffaa9836fa031f/.travis.yml

[coveralls]

Coverage increased (+0.06%) to 77.953% when pulling 1e573e2 on fix-miniconda-failures into 831ef84 on master.

[tyarkoni]

LGTM (I'm fine merging this change, whether or not it solves the actual problem)

[adelavega]

Cool. I restarted the tests one final time, to test if it uses the cache properly. I tested without cache already and it worked. So if this passes I'm pretty sure it fixes the problem and I'll merge.

[adelavega]

Looks like its working. Travis failure seems to be something else (timed out, but after env set up)