Fix conda to a particular release

[agjohnson]

Raised in readthedocs/readthedocs.org#2651

We are installing conda from a 'latest' shell script, so things go about as you'd expect. We should pin to a particular version on the docker image install step. The other images are here:

https://repo.continuum.io/miniconda/

This shouldn't require a separate update step, just pinning the version we're using to a known good version.

4.2.13 is the minimum working version, based on the ticket above.

[agjohnson]

We're now pinning the version in 2.x and latest