Travis CI: Us flake8 to find syntax errors and undefined names
Travis CI: Us flake8 to find syntax errors and undefined names

flake8 testing of https://github.com/nteract/papermill on Python 3.7.0

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./papermill/translators.py:19:15: F821 undefined name 'PapermillException'
        raise PapermillException(
              ^
1     F821 undefined name 'PapermillException'
1
```
nteract/papermill,nteract/papermill