appveyor: skip two redundant builds to speed up CI on Windows

Skip the Appveyor 32-bit Windows Server 2016 job, and 64-bit Windows
Server 2012 R2 job. The remaining 2 jobs still provide coverage of
64/32-bit, and Windows Server 2016/2012 R2.

Also set fast_finish, so failed jobs terminate the build immediately.

Fixes bug 29601; bugfix on 0.3.5.4-alpha.