Drop support for VS2013

This was scheduled for today - 20th July 2020.

Updates Appveyor configs to use VS2015 instead.

README.md

@@ -1,8 +1,14 @@
 # News
 
-1. The versioning scheme is being improved, and you might notice some differences.  This is currently WIP, but will be coming soon.  See, for example, PR #2277.
+1. Visual Studio 2013 is no longer supported
 
-2. If you get a new **compilation error due to a missing header**, it might be caused by this planned removal:
+   [As scheduled](https://github.com/KhronosGroup/glslang/blob/9eef54b2513ca6b40b47b07d24f453848b65c0df/README.md#planned-deprecationsremovals),
+Microsoft Visual Studio 2013 is no longer officially supported. \
+   Please upgrade to at least Visual Studio 2015.
+
+2. The versioning scheme is being improved, and you might notice some differences.  This is currently WIP, but will be coming soon.  See, for example, PR #2277.
+
+3. If you get a new **compilation error due to a missing header**, it might be caused by this planned removal:
 
 **SPIRV Folder, 1-May, 2020.** Glslang, when installed through CMake,
 will install a `SPIRV` folder into `${CMAKE_INSTALL_INCLUDEDIR}`.
@@ -16,11 +22,6 @@ If people are only using this location to get spirv.hpp, I recommend they get th
 [![Build Status](https://travis-ci.org/KhronosGroup/glslang.svg?branch=master)](https://travis-ci.org/KhronosGroup/glslang)
 [![Build status](https://ci.appveyor.com/api/projects/status/q6fi9cb0qnhkla68/branch/master?svg=true)](https://ci.appveyor.com/project/Khronoswebmaster/glslang/branch/master)
 
-## Planned Deprecations/Removals
-
-1. **Visual Studio 2013, 20-July, 2020.** Keeping code compiling for MS Visual Studio 2013 will no longer be
-a goal as of July 20, 2020, the fifth anniversary of the release of Visual Studio 2015.
-
 # Glslang Components and Status
 
 There are several components: