travis: Use the generic VM to test `conda-smithy`.
travis: Use the generic VM to test `conda-smithy`.

We don't actually care what VM we get from Travis CI or what comes with
it as we install Miniconda and use its Python and assorted packages for
constructing a testing environment. As a result, it would be better if
we use one of the "cheap" Travis VM images to start as we don't care
what comes with it. Here we propose using the `generic` image, which we
have used successfully in a lot of other cases. Also it has very fast
boot times. This should be helpful for iterating quickly on proposed
changes.

ocefpaf/conda-smithy,conda-forge/conda-smithy,ocefpaf/conda-smithy,shadowwalkersb/conda-smithy,shadowwalkersb/conda-smithy,conda-forge/conda-smithy