Add Python 3.4 to classifiers.

I tested Gunicorn with the following command on Pytohn 3.4.0:

    $ python3.4 -m venv venv34
    $ . venv34/bin/activate
    $ pip install -e .
    $ pip install -r requirements_dev.txt
    $ py.test tests/
Also, enable Python 3.4 on Travis CI.

(The "--use-mirrors" option was deprecated, so I removed it.)
Merge pull request #751 from berkerpeksag/classifiers-python3.4

Add Python 3.4 to classifiers.