Increment juju to 1.21.beta4
Merge pull request #1183 from sinzui/inc-1.21.beta4

Increment juju to 1.21.beta4

Increment the juju devel and win installer version th 1.21-beta4. We may not release this version. If no regressions are reported about beta3, we will rename this version 1.20.0.

(Review request: http://reviews.vapour.ws/r/494/)