Documentation nits.

Some nits:

* Fix another libgtkspell-3-0 reference for Debian Wheezy.
* Remove unnecessary `go install`s (`go get` already installs into `$GOPATH/bin`).
* Tails is consistently called Tails instead of TAILS in the official documentation (akin to Tor and TOR), but CLI should be a proper abbreviation.
* Also only use a `go get` on Tails; the `go get` documentation doesn't list the `-tags` flag, but

    `Get also accepts all the flags in the 'go build' and 'go install' commands,
    to control the installation. See 'go help build'.`