update Miniconda2 to Miniconda3

[jGaboardi]

Failures in Python 3.5 may be remedied by swapping out Miniconda2 for Miniconda3 with building TravisCI test environments.

This was part of the solution for correcting the segmentation fault: code 139 error in pysal/splot that we were seeing.

[jorisvandenbossche]

I don't think the failures are related to that. Because we create a new environment with a specific python version anyway (with https://github.com/darribas/contextily/blob/master/.travis.yml#L19).

But, that said, it is certainly a good idea to switch to download the Miniconda3 version.

[jGaboardi]

@jorisvandenbossche yep, it did not completely fix the issue as it did in splot. Now rasterio seems to causing issues, both with and without - conda config --set channel_priority strict

[jGaboardi]

related to #54

[jorisvandenbossche]

To repeat what I said above: updating to Miniconda3 is a good idea, but it is not related to testing of python 3.7 or not (since we create new environments anyway and don't use the base miniconda install)

[jGaboardi]

Miniconda update resolved.