fix(wording): clarify compass/scss feature prompt

- this better clarifies what experience the user can expect if they
answer yes to the compassBootstrap prompt, specifically that the .scss
(not .sass) version of bootstrap will be used
- aside: I waffled on whether to include `(requires Ruby)` at the tail
end of the message, but decided against it as the default error message
the user sees when they don't have ruby installed is pretty explicit.