appveyor: bump Go to 1.13.6 on Windows

Author: karalabe

appveyor.yml

@@ -23,8 +23,8 @@ environment:
 install:
   - git submodule update --init
   - rmdir C:\go /s /q
-  - appveyor DownloadFile https://dl.google.com/go/go1.13.4.windows-%GETH_ARCH%.zip
-  - 7z x go1.13.4.windows-%GETH_ARCH%.zip -y -oC:\ > NUL
+  - appveyor DownloadFile https://dl.google.com/go/go1.13.6.windows-%GETH_ARCH%.zip
+  - 7z x go1.13.6.windows-%GETH_ARCH%.zip -y -oC:\ > NUL
   - go version
   - gcc --version