Disable :const_missing parser option by default

This can be re-enabled for development mode.

lib/opal/parser.rb

@@ -88,7 +88,7 @@ def parse(source, options = {})
       @source_file              =  options[:source_file] || @file
       @method_missing           = (options[:method_missing] != false)
       @arity_check              =  options[:arity_check]
-      @const_missing            = (options[:const_missing] != false)
+      @const_missing            = (options[:const_missing] == true)
       @irb_vars                 = (options[:irb] == true)
 
       @method_calls = {}