appveyor: Do the 64-bit Windows Server 2016 job first

.appveyor.yml

@@ -6,10 +6,10 @@ clone_depth: 50
 # But we compile using MinGW, not Visual Studio.
 # We use these images because they have different Windows versions.
 image:
-  # Windows Server 2012 R2
-  - Visual Studio 2015
   # Windows Server 2016
   - Visual Studio 2017
+  # Windows Server 2012 R2
+  - Visual Studio 2015
 
 environment:
   compiler: mingw