Fix build by removing flake8-rst
Seems like `flake8-rst==0.4.2` is not available in conda anymore. This is making the Travis build fail. Newer versions of `flake8-rst` are not usable yet, as they validate more things than `0.4.2` and validating our docs would fail the CI too.

We're working on fixing all the flake8-rst linting problems in the docs, and soon we'll be able to upgrade to a newer version. But for now, I think removing `flake8-rst` from the dependencies, and disabling the docs validation in the CI is the best, so Travis builds pass again.

CC: @jreback @FHaase 