Improve Linux instructions: Cover RH-based distros and remove keybinding listing.

Hey there! This PR makes the Linux instructions cover both Debian- and Red-Hat-based systems. I don't know if the package is ``ruby-dev`` on Debian-based systems, but it's definitely ``ruby-devel`` on RH-based systems.

Also, I get rid of the keybinding listing, since that's very environment-specific. I can re-add it if you want, but only certain setups have ctrl-alt-t mapped to open a terminal.