appveyor: Explicitly set the worker VM version.
appveyor: Explicitly set the worker VM version.

Aloomaio/pymssql,JimDennis/pymssql,Aloomaio/pymssql,pymssql/pymssql,Aloomaio/pymssql,JimDennis/pymssql,JimDennis/pymssql,pymssql/pymssql