CI: Disable building on Python 3.4

As can be seen in https://travis-ci.org/adrienverge/yamllint/builds/631325436?utm_source=github_status&utm_medium=notification
The dependency, pathspec, requires Python '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*' but the running Python is 3.4.8

This commit stops Travis building yamllint against 3.4 so that CI can pass again.

.travis.yml

@@ -3,7 +3,6 @@ dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
 language: python
 python:
   - 2.7
-  - 3.4
   - 3.5
   - 3.6
   - 3.7