Bump requirements to MSVC 2015.

Track the Chromium requirements. This makes our bots build with 2015 instead of
2013.

BUG=43

Change-Id: Id5329900a5d1d5fae4b5b22299ed47bc1b947dd8
Reviewed-on: https://boringssl-review.googlesource.com/7820
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/BUILDING.md b/BUILDING.md
index e111310..33ad87a 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -24,7 +24,7 @@
     by CMake, it may be configured explicitly by setting
     `CMAKE_ASM_NASM_COMPILER`.
 
-  * A C compiler is required. On Windows, MSVC 12 (Visual Studio 2013) or later
+  * A C compiler is required. On Windows, MSVC 14 (Visual Studio 2015) or later
     with Platform SDK 8.1 or later are supported. Recent versions of GCC (4.8+)
     and Clang should work on non-Windows platforms, and maybe on Windows too.