README.rst: Point AppVeyor CI badge to new project.
README.rst: Point AppVeyor CI badge to new project.

We are switching our integration with AppVeyor Windows hosted CI
provider from a free/Open Source account
(https://ci.appveyor.com/project/pymssql/pymssql) to a paid one
(https://ci.appveyor.com/project/level12/pymssql) graciously provided by
Level 12.

This allows us to have:

* Much faster worker VMs
* Parallel builds

Thanks goes to Randy Syring for his generous offer.

pymssql/pymssql,pymssql/pymssql