Pin configparser to < 5.0 to preserve Python 2 compatibility

The configparser project just recently released version 5.0, which drops all support for Python < 3.6.

From the 5.0 release notes:

For Python 2.6-3.5, use configparser < 5

This PR pins the configparser version to >=3.5,<5.0 to preserve backwards compatibility with Python 2.7.

Once this change is merged and released, it will stop breaking our build: https://travis-ci.org/github/StackStorm/st2/jobs/667134267