Adjustments to packaging and tests

[smmaurer]

This PR makes some small changes to the packaging.

I added a MANIFEST.in file that lists the changelog and the license. This will include those files in the source distribution when we put together releases for pypi (and by extension conda-forge, which gets the files from pypi). The license file in particular is required by conda-forge.

I also added a .txt file extension to the license file because it seems tidier.

No substantive changes to the code.

EDITED TO ADD:

• fixed the version number in __init__.py, which hadn't been updated for the last release (this won't show up in pypi and conda-forge though)
• streamlined the travis tests to match the standard install process (now only installs requirements that are specified in setup.py or requirements-dev.txt)
• added python 3.6 and 3.7 to test suite and affirmed support in setup.py
• fixed tests that were failing due to changes in OSM database

[sablanchard]

@smmaurer could add to this PR bump up the version in __init__.py to the current version number, it was never increased during the last release: https://github.com/UDST/osmnet/blob/master/osmnet/__init__.py

[coveralls]

Pull Request Test Coverage Report for Build 118

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.43%

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Totals
Change from base Build 102:	0.0%
Covered Lines:	464
Relevant Lines:	502

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💛 - Coveralls