[AppVeyor] Let's pin php version.

appveyor.yml

@@ -46,8 +46,8 @@ install:
     # Install neko
     - choco install neko --prerelease --ignore-dependencies -s 'https://ci.appveyor.com/nuget/neko' -y
     # Install php
-    - choco install php -y
-    - echo extension=php_openssl.dll >> C:\tools\php71\php.ini
+    - choco install php --version 7.2.0 -y
+    - echo extension=php_openssl.dll >> C:\tools\php72\php.ini
     - RefreshEnv
     # setup python
     - cmd: mklink C:\Python34-x64\python3.exe C:\Python34-x64\python.exe