Fix a false-positive with Rails/RelativeDateConstant

[jdelStrother]

Prior to this, procs would only be allowed for the top-level assignment - eg

FOO = 1.week.ago # bad
FOO = Proc.new { 1.week.ago } # good

but didn't accept cases where the Proc was nested further down, eg:

FOO = {
  weekly: Proc.new { 1.week.ago } # bad (but should be good!)
}

This patch changes the RelativeDateConstant to recursive down through
the child nodes of the const-assignment, stopping whenever a Proc is
encountered.

As a result, ranges no longer need special-casing, so I've removed the
handling for those.

[jdelStrother]

(uhh - don't commit this yet, think I just spotted something dumb)

nvm, it's fine 😛

[koic]

Thank you for the caching up! I've wrote several comments. Please squash commits into one when you're done.

[jdelStrother]

Thanks, done

[koic]

Thanks!