CI: Remove Travis hack for enum34 crashing on Python 3.6

Revert commit 8b9eab3, it is not needed anymore.

.travis.yml

@@ -14,9 +14,6 @@ install:
   - pip install .
 script:
   - if [[ $TRAVIS_PYTHON_VERSION != 2.6 ]]; then flake8 .; fi
-  # Because of https://github.com/PyCQA/flake8-import-order/issues/149
-  # otherwise tests fail with Python 3.6:
-  - pip uninstall --yes enum34
   - yamllint --strict $(git ls-files '*.yaml' '*.yml')
   - coverage run --source=yamllint setup.py test
   - if [[ $TRAVIS_PYTHON_VERSION != 2* ]]; then