Minikube both locally and on Travis failing to build docker images

[OlafSzmidt]

We had capture_output set to False and the traceback was ignored previously. Here it is now:

Traceback (most recent call last):
  File "/home/travis/build/ocadotechnology/aimmo/integration-tests/tests/test_kubernetes.py", line 21, in setUp
    self.processes = runner.run(use_minikube=True, server_wait=False, capture_output=True)
  File "/home/travis/build/ocadotechnology/aimmo/aimmo_runner/runner.py", line 56, in run
    minikube.start()
  File "/home/travis/build/ocadotechnology/aimmo/aimmo_runner/minikube.py", line 125, in start
    build_docker_images(MINIKUBE_EXECUTABLE)
  File "/home/travis/build/ocadotechnology/aimmo/aimmo_runner/minikube.py", line 67, in build_docker_images
    matches = re.finditer(r'^export (.+)="(.+)"$', raw_env_settings, re.MULTILINE)
  File "/home/travis/virtualenv/python2.7.14/lib/python2.7/re.py", line 190, in finditer
    return _compile(pattern, flags).finditer(string)
TypeError: expected string or buffer

I also found traces of Travis not being able to run minikube docker-env beforehand.

raw_env_settings turns out to be equal to None, because the following line:
raw_env_settings = run_command([minikube, 'docker-env', '--shell="bash"'], False) isn't running.

This started when we changed our driver to none few weeks back probably. This is intended behaviour and we can safely remove the code probs.

test_travis branch first push contains more traceback