documentation: `git` not installes yet

[sjanssen2]

on a fresh system, git is probably not yet installed at this moment. You might want to add it to the list of packages above?
https://github.com/qiita-spots/qiita/blame/4492c3d53027a9387931b2449a705dd3b1c8f09f/INSTALL.md#L153

[sjanssen2]

yes :-) In this case, I figure no textual explanation is necessary as I assume new devs will blindly copy and paste the listed commands. Simply adding git to the list of ubuntu packages should protect people from running into issues later. See edits of #3409

[antgonza]

Fixed on #3409