Makefile bugs for new developers

Reported by Igor Garcia (thanks!)

> 1) couldn't "make" because of missing "zipmerge" which doesn't come with my distro (Linux Ubuntu 18.04), so I'd suggest to add that as a requirement
>
> 2) "make clean" is calling "lein clean" but as that's not a leiningen project... should we remove it?
>
> 3) After installing zipmerge, "make" still fails because of a reference to a file that does not exist.
https://github.com/chr15m/slingcode/blob/master/src/slingcode/main.cljs#L47
>
> here's a piece of the output:
>
> ```
> 46 | (def logo (rc/inline "slingcode/logo.svg"))
> 47 | (def revision (rc/inline "slingcode/revision.txt"))
> ---------------------^----------------------------------------------------------
> Encountered error when macroexpanding shadow.resource/inline.
> Resource not found: slingcode/revision.txt at line 47 slingcode/main.cljs
> ```