Removed dependency of requests from okapi

Removed dependency of requests from okapi and added dependency to request_lib exceptions

@lorenzogil @gasull @haplo @rmariad 
Usually we put something like ```0.8.1 (unreleased)``` since at the time of this change we don't always know when the next release will be.

I'm just being super picky to give you more information. This is not worth a new change on this PR.