doc/guide/installation.rst: add note on inspecting go script at least once

Started with comment from @lunaryorn  https://github.com/cask/cask/commit/44105a2a4c58a6576a4a642c98213b7e7a109111#commitcomment-7895362

Sorry if it's a bit impersonal.

I also removed the lockfile of `installation.rst` that seems inadvertently committed.