Use ELPA

We've got a package of gnuplot-mode in Melpa, so you can probably tick this issue off as "done". :-)

In fact, there are two different Melpa packages: "gnuplot" is this library, "gnuplot-mode" is a different, minimalistic one (github.com/mkmcc/gnuplot-mode). That's kind of confusing, but there we go.

Yes, when the latter was submitted I did a double-take, because I was surprised anyone would not just use this gnuplot-mode.