Replace flake8 with prospector

[Prospector](http://prospector.readthedocs.org/en/latest/) is a tool that runs a number of Python linters and static analysis tools and aggregates their output. Notably, we weren't running Pylint on our code automatically, which would have caught the problems fixed in #1674.

This replaces the flake8 setup for Travis with one that runs Prospector. At the moment it should catch all of the same errors flake8 did, with the exception of naming convention errors (previously caught by [`pep8-naming`](https://pypi.python.org/pypi/pep8-naming) -- there's no easy way to reenable these as far as I can tell. Additionally, it will run a relatively lenient Pylint configuration, which will catch things such as broken imports.