Update .appveyor.yml

.appveyor.yml

@@ -1,5 +1,5 @@
 os:
-  - Visual Studio 2013
+  - Visual Studio 2017
 
 install:
   - 3rdparty\bx\tools\bin\windows\genie --file=scripts\genie.lua --with-luajit --with-openal --with-bullet --with-tools --no-level-editor vs2017