bump gives wrong version if you're 2+ steps behind published

[rtfeldman]

To reproduce:

1. Check out the code for a package that's currently published as 1.1.0
2. Edit its version in elm-package.json to be 1.0.0
3. Add something new, such that this will be a MINOR change.
4. Run elm package bump

Expected: it changes the local 1.0.0 to 1.2.0, since that's the next MINOR version to be published.

Actual: it changes the local 1.0.0 to 1.1.0, even though 1.1.0 has already been published.

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.