Drop support for Python 3.4 in CI builds

Python 3.4 is not supported by Dan Blanchard's packaged version
nose-cov, causing Travis CI builds to fail:

    Hint: the following combinations of packages create a conflict with the
    remaining packages:
      - python 3.4*
      - nose-cov
    The command "conda install --yes -c dan_blanchard python-coveralls nose-cov" failed and exited with 1 during .
    Your build has been stopped.