refine appveyor yaml. fixes #1212

appveyor.yml

@@ -1,6 +1,8 @@
-os: Visual Studio 2017
+image: Visual Studio 2017
 
-platform: x64
+platform:
+  - x64
+  - x86
 
 branches:
   only:
@@ -17,6 +19,10 @@ install:
   - go get -u v2ray.com/core/...
   - go get -u v2ray.com/ext/...
 
+build_script:
+  - go build v2ray.com/core/main
+  - go build v2ray.com/ext/tools/control/main
+
 build: off
 
 test_script: