appveyor: bump Go to 1.9.2 (ethereum#15726)

Author: karalabe

appveyor.yml

@@ -23,8 +23,8 @@ environment:
 install:
   - git submodule update --init
   - rmdir C:\go /s /q
-  - appveyor DownloadFile https://storage.googleapis.com/golang/go1.9.windows-%GETH_ARCH%.zip
-  - 7z x go1.9.windows-%GETH_ARCH%.zip -y -oC:\ > NUL
+  - appveyor DownloadFile https://storage.googleapis.com/golang/go1.9.2.windows-%GETH_ARCH%.zip
+  - 7z x go1.9.2.windows-%GETH_ARCH%.zip -y -oC:\ > NUL
   - go version
   - gcc --version