Update README to confirm PostgreSQL 12 support.

Author: pitfield

README.md

@@ -93,7 +93,7 @@ PostgresClientKit is compatible with Linux, macOS, and iOS.  It has been tested
 - Ubuntu 18.04 LTS 
 - macOS 10.14
 - iOS 12
-- Postgres 10, 11, 12beta1
+- Postgres 10, 11, 12
 
 ## Building