Appveyor: switch master to VS 2019

Author: cmb69

.appveyor.yml

@@ -1,6 +1,6 @@
 version: "{branch}.build.{build}"
 
-image: Visual Studio 2017
+image: Visual Studio 2019
 
 branches:
         except:
@@ -24,7 +24,7 @@ environment:
         PHP_BUILD_OBJ_DIR: c:\obj
         PHP_BUILD_CACHE_SDK_DIR: c:\build-cache\sdk
         PHP_BUILD_SDK_BRANCH: php-sdk-2.2.0
-        PHP_BUILD_CRT: vc15
+        PHP_BUILD_CRT: vs16
         # ext and env setup for tests
         #MYSQL_TEST_PASSWD: Password12!
         #MYSQL_TEST_USER: root