Enable rubocop

This setup of rubocop is very lenient to begin with. Some leniencies are established in .rubocop.yml and other times rules are disabled inline in the files. Over time, more rules should be enabled.

This also adds rubocop to the default rake task.

I haven't gone back and fully reviewed everything myself, but I plan to this week. It's also probably better to get more eyes on this as early as possible given how big the change is.

I also should note that the bulk of these conversions were done by Rubocop's auto-converter.

Addresses #1586.
`$ERROR_INFO` comes from the `English` standard library and we don't load that library.  Nor do we want to...we try to load as few stdlib pieces as possible so that users don't get false positives where there code passes tests but fails in prod due to requires they forgot (which weren't needed in their test environment since rspec had already loaded the library).

I'd prefer to disable whichever cop suggested you use the `English.rb` aliases.