Fixes the failing pip-chill command and Travis build

[NdagiStanley]

What does this PR do?

Fixes the failing pip-chill command execution

Description of Task to be completed?

• Fix the error that occurs when pip-chill is run
• Fix the tests on Travis

  How should this be manually tested?

  First off activate a python environment then run:

  python setup.py install
  pip-chill

  The output should have no error.

  Any background context you want to provide?

• @LevN0 explains what's up with Travis as far as Python 3.3 is concerned here.
• I have solved @ebrensi's PR #15 issue of the tox tests failing in Python 3.3
• This StackOverflow thread led me to the alternative of pip.get_installed_distributions which is no longer contained in pip 10.0.0 version onwards