Python 3.3 and 3.5 not tested under Travis

[vladimir-v-diaz]

Description of issue or feature request:
Pull request #59 temporarily excluded testing under Python versions 3.3 and 3.5 due to issues with Travis. Although testing under Tox does cover all of our supported Python versions, it is best if continuous integration covered all supported environments. As recommended in #59 (comment), we should open a ticket to keep track of this issue.

Current behavior:
Travis runs our unit tests under Python versions 2.7, 3.4, and 3.6.

Expected behavior:
Travis should run our unit tests under Python versions 2.7, 3.3, 3.4, 3.5, and 3.6.