Add rasterio, gdal, python versions to the circleci build matrix

[perrygeo]

Manually testing these permutations is a pain and it would be nice to ensure this (and other rio plugins) work across the board. Need to figure out the best way to make that happen with CircleCI.

[perrygeo]

I figured out how to test across python versions (2.7, 3.4, 3.5) on CircleCI with tox and pyenv. See https://github.com/mapbox/raster-tester/pull/32/files

However, the concept of a build matrix is a bit foreign to CircleCI. If we need to test multiple Rasterio and GDAL versions, we would be swimming against the current. That type of setup (all permutations of different versions) is relatively simple in Travis.

Do we need to test different GDAL and Rasterio versions? Many of us are testing GDAL 2 locally and we'll soon all be on the Rasterio 1.x train. So maybe sticking with CircleCI/tox/pyenv for python version testing is OK for now?

cc @dnomadb @virginiayung @sgillies

[virginiayung]

Nice! @perrygeo I did try ☝️ the tox method here and it worked for the rio-alpha repo as well, but my problem in the end was with pyenv virtualenv not playing nicely with Matplotlib. Any suggestions with that? Thanks! Otherwise I'm happy with Travis :)

[sgillies]

@perrygeo let's let the Rasterio repo take all the responsibility for the GDAL dimension of the testing matrix. Raster-tester can then stick with one version. I'm pretty new to tox and not sure how one would use it to vary system libs.