travis test fails

[nathanlyons]

Some test are failing for #18. The error looks like: python[version='>=3.6,<3.7.0a0'].

@mcflugen, is this a similar issue that was resolved in landlab/landlab#633?

[mcflugen]

@nathanlyons Yes, I'm pretty sure.

landlab needs to be installed into an environment and the tests run in that environment.

So .travis.ym should have something like

- conda create -n _testing python=$TRAVIS_PYTHON_VERSION
- source activate _testing

instead of

- conda install python=$TRAVIS_PYTHON_VERSION

The problem is that the latest version of miniconda installs Python 3 into the root environment, which means that if we want to install a different version of Python, we have to install it into a separate environment (this used to not be the case).

[nathanlyons]

@mcflugen Thanks. Do you think #21 might address this issue? The issue and yml of this pull are congruent.