import error if flanker pulls in regex==2018.02.19 as a dependency

[wgwz]

https://bitbucket.org/mrabarnett/mrab-regex/issues/314/import-error-no-module-named https://github.com/mailgun/flanker/blob/master/setup.py#L41

if you upgrade to regex==2018.02.19, import regex as re fails. i think the resolution needs to be upstream, this is more just a post in case it's helpful for others.