Replace `or` with `||` and `not` with `!` in README

All the popular ruby style guides ban the `or` keyword:

> The `and` and `or` keywords are banned. It's just not worth it. Always use
> `&&` and `||` instead.

https://github.com/styleguide/ruby
https://github.com/bbatsov/ruby-style-guide#no-and-or-or
https://github.com/airbnb/ruby#conditional-keywords

The same idea applies to `not`: It's for control flow, not boolean logic.
And like the others, its use here is misleading to programmers coming from
Python and Coffeescript.