Elm CI tests improvements

[PeterDSteinberg]

• Try out https://github.com/tarpas/pytest-testmon/ for running pytest on the subset of code affected by changes. Note we have a temporary fix in place here in test_pipeline.py from PR Cross validation of Pipeline/estimators using MLDataset / xarray.Dataset #221
• See if anything (channels, git vs conda) can be done to simplify the CI systems and interactions of:
  • elm
  • xarray_filters - xarray_filters PR 45 added auto-package uploads
  • earthio
  • dask-searchcv
• Make sure the .travis.yaml is uploading to anaconda.org under the dev channel of elm user. Earthio and Elm have both had bugs recently in the deploy section of .travis.yml