Update Travis script

[smmaurer]

This PR updates the Travis script, to simplify it and run tests in newer Python environments. Also updates some other ancillary things that came up in the process.

Travis script

Old version: .travis.yml
New version: .travis.yml

This runs the same code style and unit tests, removing some outdated cruft -- replaces a Conda custom environment with pip install ., drops an old slack notification system, etc. Tests now run in Python 2.7, 3.5, 3.6, 3.7, and 3.8.

Python 3.6+ will use current versions of Pandas and NumPy, but earlier environments will use older versions, which is helpful for maintaining backward compatibility.

Pandana syntax

Running the tests in newer environments raised a few errors related to Pandas deprecations, which I fixed:

• pd.Series.nonzero() -> pd.Series.values.nonzero()
• pd.Series.as_matrix() -> pd.Series.values()
• pd.Index.name = -> pd.Index.set_names()

Other changes

• added a requirements-dev.txt file to list the additional packages used for testing in a central place
• switched the code style checks from pep8 to pycodestyle

[msoltadeo]

Looks ok to me

[coveralls]

Coverage remained the same at 82.451% when pulling 35e8b79 on update-travis into f8dbfc4 on develop.

[PyMap]

@smmaurer pandas updates are fine to me! I will approve for those ones and let you decide regarding the travis checks! Thanks for accomodating this!