[AppVeyor] fix php installation

appveyor.yml

@@ -36,10 +36,10 @@ install:
     - '%CYG_ROOT%/bin/bash -lc "opam install camlp4 --yes"'
     # Install neko
     - choco install neko --prerelease --ignore-dependencies -s 'https://ci.appveyor.com/nuget/neko' -y
-    - choco install php --ignore-dependencies
-    - echo extension=php_mysqli.dll >> C:\tools\php\php.ini
-    - echo extension=php_sqlite3.dll >> C:\tools\php\php.ini
-    - echo extension=php_openssl.dll >> C:\tools\php\php.ini
+    - choco install chocolatey-core.extension php --ignore-dependencies -y
+    - echo extension=php_mysqli.dll >> C:\tools\php71\php.ini
+    - echo extension=php_sqlite3.dll >> C:\tools\php71\php.ini
+    - echo extension=php_openssl.dll >> C:\tools\php71\php.ini
     - RefreshEnv
     - neko -version
     # setup python