Replace Rails method with Ruby equivilent

There was a crash when accessing hashes while not inside a rails
environment. This was due to us accidentally using a String method
which is only available in Rails.

lib/awesome_print/formatters/hash_formatter.rb

@@ -47,7 +47,7 @@ module AwesomePrint
       private
 
       def symbol?(k)
-        k.first == ':'
+        k[0] == ':'
       end
 
       def ruby19_syntax(k, v, width)