appveyor.yml: Avoid pre-release of coala-bears
The pre-release of coala-bears was being installed on appveyor, which was causing the build to fail mainly due to ApertiumLintBear in the bears 0.12 pre-release also, Also undo the VintBear workaround, which is unnecessary after 0.11.1.
Fixes #143 (closed)
Edited by Monal Shadi