BUG: python 3.5 unit tests and xarray

[jklenzing]

Description

xarray 0.14.0 is not compatible with python 3.5. This updates the environment setup in Travis CI to manually install both xarray and pandas (which is required by xarray and will create errors down the line if not installed manually in the environment).

Also updated the custom pysat install to develop since the no_sgp4 branch is merged there.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Updates to Travis CI environment, so tested there. Based on lessons learned from sami2py/sami2py#81 and pysat/pysatMissions#7.

Checklist:

• Make sure you are merging into the develop (not master) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Add a note to CHANGELOG.md, summarizing the changes

[jklenzing]

Looking over pysat/pysatMissions#7, also implemented changes to the directory structure based on pysat/pysat#307. This will prevent future headaches when more unit tests are added, or when people try to develop from a fork.