Require Visual Studio 2015 or later.

Something about Visual Studio 2013 causes test failures,
but I currently have no idea what that could be because
there is no output from the failed tests whatsoever. :(

Change-Id: Ic57cba54115ededa738ed42f4fffe5dd2d6350de
Reviewed-on: https://code-review.googlesource.com/20930
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 884873b..702a7ec 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -22,8 +22,8 @@
 set(EXTRA_TARGET_LINK_LIBRARIES)
 
 if(CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
-  if(MSVC_VERSION LESS 1800)
-    message(FATAL_ERROR "you need Visual Studio 2013 or later")
+  if(MSVC_VERSION LESS 1900)
+    message(FATAL_ERROR "you need Visual Studio 2015 or later")
   endif()
   if(BUILD_SHARED_LIBS)
     # See http://www.kitware.com/blog/home/post/939 for details.