doc: update windows prerequisites

Visual Studio 2013 is no longer supported. Update the Windows build prerequisites. Refs: #7484 PR-URL: #8049 Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: João Reis <reis@janeasystems.com>

Author: bnoordhuis

BUILDING.md

@@ -101,8 +101,8 @@ Prerequisites:
 * [Python 2.6 or 2.7](https://www.python.org/downloads/)
 * One of:
  * [Visual C++ Build Tools](http://landinghub.visualstudio.com/visual-cpp-build-tools)
- * [Visual Studio](https://www.visualstudio.com/) 2013 / 2015, all editions including the Community edition
- * [Visual Studio](https://www.visualstudio.com/) Express 2013 / 2015 for Desktop
+ * [Visual Studio 2015 Update 3](https://www.visualstudio.com/), all editions
+  including the Community edition.
 * Basic Unix tools required for some tests,
  [Git for Windows](http://git-scm.com/download/win) includes Git Bash
  and tools which can be included in the global `PATH`.