Travis CI: Add Python 3.7 and flake8 to the testing

[cclauss]

Add flake8 tests to find Python syntax errors and undefined names.

E901,E999,F821,F822,F823 are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety.

• F821: undefined name name
• F822: undefined name name in __all__
• F823: local variable name referenced before assignment
• E901: SyntaxError or IndentationError
• E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

[x4v13r64]

Thanks for the PR. Closing as Scout2 is no longer under development.

These changes have been branched into https://github.com/nccgroup/ScoutSuite/tree/update_travis and will be merged into Scout Suite' master branch as soon as testing is setup.