Use String#unpack instead of String#codepoints.

The latter returns an Enumerable which on Ruby 1.9.3 doesn't have #length
available. Besides this it's better to just return an Array since we'll iterate
over every character anyway.

lib/oga/lexer.rl

@@ -95,7 +95,7 @@ module Oga
     # @return [Array]
     #
     def lex(data)
-      @data       = data.codepoints
+      @data       = data.unpack('U*')
       lexer_start = self.class.lexer_start
       eof         = data.length