New release (candidate): 1.2.0-RC2

Since there were some minor issues reported with the RC1 (concerning backwards compatibility to older versions of setuptools and pip that are still present on a large number of systems out there) I fixed those and just tagged RC2.

See the changelog here: https://github.com/foosel/OctoPrint/releases/tag/1.2.0-rc2

The full changelog for 1.2.0 can be found here: https://github.com/foosel/OctoPrint/blob/1.2.0-rc2/CHANGELOG.md#120-unreleased

Please test this and report back. I want to release the final 1.2.0 version next week, and I'd prefer if I didn't have to do a bugfix release immediately after, so any reports I get now help to get 1.2.0 as stable as possible for general consumption.