Simplify test for divisibility by all numbers 1…20
Ruby refactor: use Enumerable.all?:

def divisible?(n)
  (1..20).all? { |x| n % x == 0 }
end


Mathematical refactor:calculate the least common multiple of the integers in the range (Integer#lcm is available from Ruby 1.9):

def divisible?(n)
  n % (1..20).reduce(:lcm) == 0
end


This second snippet is, of course, more efficient once you pre-calculate (1..20).reduce(:lcm) only once and store it somewhere.