Update Emacs instructions to use MELPA
Update Emacs instructions to use MELPA

import-js.el is now available on MELPA, so that should be how most users
install.

lencioni/import-js,lencioni/import-js,Galooshi/import-js,trotzig/import-js,lencioni/import-js,trotzig/import-js,trotzig/import-js