Drop Python 3.4 support

[tdenewiler]

Statick transitively depends on the pathspec package (via yamllint), which dropped support for Python 3.4 in their latest release this week: 0.7.0. The work was done in cpburnz/python-pathspec#28. This is causing our Travis jobs to fail (see #165) on the Python 3.4 job.

I am going to drop support for testing against Python 3.4. It was end-of-lifed 2019-03-18 (https://www.python.org/downloads/release/python-3410/). If there is a desire by users to keep supporting Python 3.4 we could do something along the lines of trying to have that Travis job use a pinned version of the pathspec package at version 0.6.0. That seems like it might cause more issues requiring more and more package pinning over time.

[denewiler]

Done in #167.