Fix appveyor builds.

Author: stefanseefeld

.appveyor.yml

@@ -30,7 +30,7 @@ environment:
       PYTHON: C:\\Python36-x64
       PYTHON_VERSION: 3.6.x
       PYTHON_ARCH: 64
-      MSVC: 16.6.5
+      MSVC: 16.7.4
       ARCH: x86_64
       BOOST_PREFIX: C:\Libraries\boost_1_73_0