install RVM with GPG signatures (per rvm.io)

[alexch]

see #504

Does this mean we may need an "install gpg" step too?

[ultrasaurus]

On my OSX laptop, I get -bash: gpg: command not found when I run the new command, so yeah, it can't go in as-is. Would also love an explanation blurb on some of this stuff with links to what it means -- neat opportunity to teach people a little about security while they are waiting for downloads, for the folks who are ready/interested.

[tjgrathwell]

Added it instead as a conditional step instead in 9a6adee

RVM only demands you add the GPG keys if you have gpg or gpg2 installed, which seems to only affect a small proportion of students (those who have other programmery-type stuff installed on their computer already). But it's still important that they run the extra command if they see the error 😸