Arrowhead

Code fixes based on comments and rubocop.
Thinking about the method names, what about this formulation?

```
def self.classify(region, shape)
  arrowhead = shapes_for(region).fetch(shape)
  puts "You have a(n) '#{arrowhead}' arrowhead. Probably priceless."
rescue KeyError
  raise "Unknown. Are you sure you know what you're talking about?"
end

def self.shapes_for(region)
  CLASSIFICATIONS.fetch(region)
end
```