bump requests version in dev environment

[jhamman]

This simply bumps the version of the requests library in the dev environment. I'm doing this because GitHub keeps sending me emails about a known security vulnerability in requests<= 2.19.1. This satisfy that issue.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• Docs build locally (e.g., run tox -e docs)
• AppVeyor and Travis CI passes
• Test coverage is 100% (Coveralls passes)

[alimanfoo]

Thanks @jhamman, this should already be covered in PR #326, we should try and get that merged.