Install current version of Go

This is inspired by the appveyor.yaml from the go-plus project:
https://github.com/joefitzgerald/go-plus/blob/master/appveyor.yml

appveyor.yml

@@ -3,7 +3,17 @@ clone_folder: c:\gopath\src\github.com\restic\restic
 environment:
   GOPATH: c:\gopath;c:\gopath\src\github.com\restic\restic\Godeps\_workspace
 
+init:
+  - ps: >-
+      $app = Get-WmiObject -Class Win32_Product -Filter "Vendor = 'http://golang.org'"
+
+      if ($app) {
+        $app.Uninstall()
+      }
+
 install:
+  - appveyor DownloadFile https://storage.googleapis.com/golang/go1.5.1.windows-amd64.msi
+  - msiexec /i go1.5.1.windows-amd64.msi /q
   - go version
   - go env
   - appveyor DownloadFile http://downloads.sourceforge.net/project/gnuwin32/tar/1.13-1/tar-1.13-1-bin.zip -FileName tar.zip