Use 'v140' msbuild_toolset for VS project.

We use /SUBSYSTEM:Windows and 'MinimumRequiredVersion': '5.01' to
make our app work in Windows XP, but we use 'v140' toolset instead
of 'v140_xp' so that we have access to modern APIs like WRL.

Telegram/gyp/telegram_win.gypi

@@ -21,7 +21,7 @@
   'conditions': [[ 'build_win', {
     'msvs_precompiled_source': '<(src_loc)/stdafx.cpp',
     'msvs_precompiled_header': '<(src_loc)/stdafx.h',
-    'msbuild_toolset': 'v140_xp',     #Windows7.1SDK
+    'msbuild_toolset': 'v140',
     'sources': [
       '<(res_loc)/winrc/Telegram.rc',
     ],