Remove duplicate license parameter

[blag]

The Python packaging specifications indicate that the license parameter to setup() should be used in one of the following cases:

• the license is not a selection from the “License” Trove classifiers
• to specify a particular version of a license which is named via the Classifier field
• to indicate a variation or exception to such a license

This PR removes the explicit license parameter since the usage for this project does not fit any of those cases and it makes it more difficult to use tools like pip-licenses since it is not consistent with other projects that just have License :: OSI Approved :: MIT License as one of their trove classifiers.

Please note that this does not change the license of the project in any way, and since this project already has a License :: OSI Approved :: MIT License trove classifier the license parameter is redundant.

[blag]

The Travis CI failures are due to old versions of Python, not due to this change.