add os to appveyor.yml
diff --git a/appveyor.yml b/appveyor.yml
index 990e8e6..af8aad8 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,3 +1,5 @@
+os: Visual Studio 2015
+
 build_script:
   - mkdir build
   - cd build