Bump minimum xarray version to 0.13.0

[johnomotani]

I propose bumping the minimum version of xarray to 0.13.0. I want to use the 'join' kwarg for concat which was introduced in 0.13.0.

[TomNicholas]

I'm using 0.14.1 so I expect this is fine

[ZedThree]

Just noticed the list of packages for Travis -- is this just duplicating requirements.txt/setup.py? Could just pip install -r requirements.txt?

[johnomotani]

Just noticed the list of packages for Travis -- is this just duplicating requirements.txt/setup.py? Could just pip install -r requirements.txt?

One of the builds on Travis does do pip install -r requirements.txt. The explicit PIP_PACKAGES=... is for a second version that uses the minimum required version of each package. Would be nice if there's a neater way to do this, but pip is keener on giving you the newest possible version of each package!