No job_start in earlier versions of vim

[sarahscheffler]

I originally tried to use this in vim 7.4.898 (this is apparently still default in OSX Sierra), but that failed when opening a .md file on line 43 of composer.vim with the error E117: Unknown function: job_start.

Updating to Vim 8.0 fixed the problem, but maybe this should be documented somewhere? (I'm not sure about versions between 7.4 and 8.0, if there are any.)

[euclio]

Ah, whoops, I totally had this in an earlier revision of the README, but I guess I never actually committed it. Yeah, Vim 8 is required.