Update CI to use GitHub Actions

[lwasser]

closes #630

This updates CI to use actions rather than travis

a few notes

1. Allow option to setup my own environment with environment.yml s-weigand/setup-conda#6 recommends using source activate given conda init requires a bash restart which is tricky. The alternative would be the other conda action which using a $conda envt and path to conda for each call - that seems clunky so going with source activate
2. the install is WAY slower with conda vs using pip (look at abc-classroom actions) but i think we need to given the many conda dependencies.

We don't need to lint and build docs on each version so i pulled them out into a separate build. this simplifies the conda environment and that part of the build will be speedy! seems like a win all around. We could consider adding code cov to that build in the future.

[lwasser]

this is failing because of appveyor. i've gone ahead and turned off appveyor completely so it wont fail for future builds! all other tests and runs are passing. merging this.