back-port start_with? from 1.8.7

This allows its use in Leopard, which has Ruby 1.8.6.

Library/Homebrew/extend/string.rb

@@ -2,6 +2,13 @@ class String
   def undent
     gsub(/^.{#{slice(/^ +/).length}}/, '')
   end
+
+  unless String.method_defined?(:start_with?)
+    def start_with? prefix
+      prefix = prefix.to_s
+      self[0, prefix.length] == prefix
+    end
+  end
 end
 
 # used by the inreplace function (in utils.rb)