Use a more recent vala-mode than ELPA's; remove magithub; fix dtrt-indent; add instructions for replicating a configuration

I give up on ELPA, it's harder to get things into than Emacs itself!

magithub is obsolete (I've cited the relevant magit issue), so remove its recipe. Note that it's obsolete in a strong sense: it does not work any more with github, AND it stops magit itself from working, owing to using an out-of-date variable name.