Refresh a lot of the copy

This includes some small and some larger(er) changes, which I'll call out specifically in a moment, but generally:

* Typos and wording (small)
* Bring some tech stuff up to date (small)
* Change a few things about the rails engineering job description (bigger)
When I tell people we do a monthly gem update, that seems to be really appealing. So maybe leave out version numbers and just mention. "part of our light-weight process is a monthly "gem update", to fight off technical debt."