appveyor: Removed VS2013, added VS2017.

.appveyor.yml

@@ -1,13 +1,12 @@
 shallow_clone: true
 
 os:
-  - Visual Studio 2015
+  - Visual Studio 2017
 
 environment:
   matrix:
-  - TOOLSET: vs2012
-  - TOOLSET: vs2013
   - TOOLSET: vs2015
+  - TOOLSET: vs2017
 
 configuration:
   - Debug