Add option to test 32 bit builds

[kain88-de]

We have now an issue that our package doesn't pass the tests on 32 bit platforms. We haven't noticed so far but this is still an issue for people who want to package a python package since most distributions haven't dropped 32 bit support yet. The solution would be to download the 32bit version of conda. See obspy for how to do it.

[astrofrog]

@kain88-de - we do 32-bit testing for Astropy via CircleCI (we don't use ci-helpers). We have a docker image for that. We could probably document this here or at least point to it. But I'll take a look at obspy since I hadn't thought of the 32-bit conda.

[bsipocz]

If there is anyone volunteering to do it we could probably add some kind of CircleCI support here. I believe there is already an issue for it (#112)

[kain88-de]

@jbarnoud is currently testing a travis version. (MDAnalysis/mdanalysis#1392)

[kain88-de]

There is also a longer discussion of adding unix 32bit support for conda-forge conda-forge/conda-forge.github.io#163

[jbarnoud]

I am working on it in this branch: https://github.com/jbarnoud/ci-helpers/tree/32bits
I'll make a pull request if it ends up working, but so far I have some trouble compiling C extensions for 32 bits python on 64 bits travis.