[Twisted-Python] Enable rolling builds for Appveyor

Adi, Can you enable "rolling builds" on Appveyor: https://www.appveyor.com/docs/build-configuration/#rolling-builds If someone pushes a new commit to a branch, that will stop old builds currently in-progress or queued for the branch, and just queue the new commit for the branch. This can be done on the settings page at: https://ci.appveyor.com/project/adiroiban/twisted/settings -- Craig