padded.rb

#!/usr/bin/env ruby

def padded_tweet(message)
  if message.size >= 140
    message[0..139]
  elsif %w{. ! ? …}.include? message[-1]
    message.ljust(140, message[-1])
  else
    message.ljust(140, '!')
  end
end

puts padded_tweet(ARGV.first)