appveyor: Force Visual Studio 2013 image.

It seems the default build image is now Visual Studio 2015, and Visual
Studio 2013 is not installed.

diff --git a/appveyor.yml b/appveyor.yml
index 2618f69..c2efa7e 100644 (file)
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -37,6 +37,8 @@ cache:
 - win_flex_bison-2.4.5.zip
 - llvm-3.3.1-msvc2013-mtd.7z
 
+os: Visual Studio 2013
+
 environment:
   WINFLEXBISON_ARCHIVE: win_flex_bison-2.4.5.zip
   LLVM_ARCHIVE: llvm-3.3.1-msvc2013-mtd.7z