Fixes for Travis CI

- Remove `--ignore-pipfile`. This option makes Pipenv install from the
  `Pipfile.lock` only, ignoring the `Pipfile` itself. In a sense this is
  good, because it tests with the locked dependencies. However, it
  breaks when you lock on Python 3.6 but test on Python 3.4 or 2.7.
  We'll have to re-visit this when dropping support for older Pythons.

- pypy 3.6 -> 3.5

- Drop `pypy` from Travis CI testing
  We still test with 'pypy3.5'. However, Pipenv has an issue with pypy 2.7.
  See https://github.com/pypa/pipenv/issues/2449

- Pypy3.5 seems to want pathlib2 when running on Travis CI