Generate a universal wheel in addition to the source distribution

[edmorley]

See:
https://packaging.python.org/tutorials/distributing-packages/#universal-wheels
https://docs.travis-ci.com/user/deployment/pypi/#Uploading-different-distributions

Fixes #219.

[coveralls]

Coverage remained the same at 92.356% when pulling 9fbb183 on edmorley:generate-wheels into cec1a84 on graphql-python:master.

[coveralls]

Coverage remained the same at 92.356% when pulling 9fbb183 on edmorley:generate-wheels into cec1a84 on graphql-python:master.

[edmorley]

Many thanks :-)