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>

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.
 

util/bot/vs_toolchain.py

@@ -64,8 +64,8 @@ def SetEnvironmentAndGetRuntimeDllDirs():
 def _GetDesiredVsToolchainHashes():
   """Load a list of SHA1s corresponding to the toolchains that we want installed
   to build with."""
-  # Use Chromium's VS2013.
-  return ['4087e065abebdca6dbd0caca2910c6718d2ec67f']
+  # Use Chromium's VS2015.
+  return ['95ddda401ec5678f15eeed01d2bee08fcbc5ee97']
 
 
 def FindDepotTools():