users should remove former Go installation folder

doc/install/installation.md

@@ -142,6 +142,9 @@ gitlab-workhorse we need a Go compiler. The instructions below assume you
 use 64-bit Linux. You can find downloads for other platforms at the [Go download
 page](https://golang.org/dl).
+    # Remove former Go installation folder
+    sudo rm -rf /usr/local/go
+
     curl --remote-name --progress https://storage.googleapis.com/golang/go1.5.3.linux-amd64.tar.gz
     echo '43afe0c5017e502630b1aea4d44b8a7f059bf60d7f29dfd58db454d4e4e0ae53  go1.5.3.linux-amd64.tar.gz' | shasum -a256 -c - && \
       sudo tar -C /usr/local -xzf go1.5.3.linux-amd64.tar.gz