color-theme-solarized: update and fix

I suspect there is something wrong with the elpa package generator. Emacs'
setup-hook adds the `site-lisp` directory to `load-path`, but elpa packages end
up having their code in `site-lisp/elpa/package-version, and thus emacs cannot
load them.

If that worked, adding `color-theme` as `builInput` might be better than
explicitly adding its `site-lisp` to the `load-path` in the `buildPhase`