[AppVeyor] use php 7.1.8

Unit test failed when using php 7.1.9/7.1.10... no idea why.
```
Command: php [bin/php7/index.php]
TestMain.hx:36: Generated at: 2017-10-03 10:40:52
TestMain.hx:38: START
Math.unit.hx:76: false should be true
Math.unit.hx:77: false should be true
Math.unit.hx:79: false should be true
Math.unit.hx:80: false should be true
Math.unit.hx:101: false should be true
Math.unit.hx:102: false should be true
Math.unit.hx:103: false should be true
Math.unit.hx:104: false should be true
Math.unit.hx:301: false should be true
Math.unit.hx:302: false should be true
Math.unit.hx:304: false should be true
Math.unit.hx:305: false should be true
Math.unit.hx:326: false should be true
Math.unit.hx:327: false should be true
Math.unit.hx:328: false should be true
Math.unit.hx:329: false should be true
Test.hx:220: DONE [6856 tests]
Test.hx:221: SUCCESS: false
Command exited with 1 in 1s: php [bin/php7/index.php]
```

appveyor.yml

@@ -33,6 +33,8 @@ cache:
 install:
     - 'git submodule update --init --recursive'
     - '%CYG_ROOT%/bin/bash -lc "echo initialize"'
+    # http://help.appveyor.com/discussions/problems/5616-not-able-to-build-due-to-problem-in-chocolateyinstallps1
+    - ps: Set-Service wuauserv -StartupType Manual
     - choco install curl
     # Install ocaml
     - curl -fsSL -o cygwin-setup.exe --retry 3 https://cygwin.com/setup-x86_64.exe
@@ -46,7 +48,7 @@ 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 chocolatey-core.extension php --ignore-dependencies -y
+    - choco install php --version 7.1.8 -y
     - choco install nsis.portable
     - choco install wget
     - echo extension=php_mysqli.dll >> C:\tools\php71\php.ini