Bump dev version to 1.9.1-dev

[timvisee]

Bumps the development version to 1.9.1-dev. We're currently releasing 1.9.0 so we should bump our development version as well.

If we eventually decide we'll do a new major release instead we can bump this to 1.10.0-dev.

All Submissions:

• Contributions should target the dev branch. Did you create your branch from dev?
• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

[timvisee]

To clarify, this is just for our dev branch. This will not reach master.