Do not test on Python 3.4 (#619)
Do not test on Python 3.4 (#619)

Python 3.4 unit tests fail with the following error:

```
nox > Running session unit_tests(python_version='3.4')
nox > /usr/local/bin/python -m virtualenv /usr/src/artman/.nox/unit_tests-python_version-3-4 -p python3.4
nox > chdir /usr/src/artman
nox > pip install --upgrade mock pytest pytest-cov pyfakefs restructuredtext_lint
nox > pip install --upgrade -e .
nox > Command pip install --upgrade -e . failed with exit code 1:
DEPRECATION: Python 3.4 support has been deprecated. pip 19.1 will be the last one supporting it. Please upgrade your Python as Python 3.4 won't be maintained after March 2019 (cf PEP 429).
```

Let's stop running unit tests on Python 3.4.
googleapis/artman,googleapis/artman,googleapis/artman