Fixed type conversion from VALUE to GType

  * bug fix for the following errors
    * clutter/lib/clutter.rb:123:in `register_boxed_class_converter': integer 140532096032688 too big to convert to `unsigned int' (RangeError)
    * gobject-introspection/lib/gobject-introspection/loader.rb:177:in `get_field_value': integer 140479269924112 too big to convert to `unsigned int'
Merge pull request #186 from tanahiro/master

Fixed type conversion from VALUE to GType

Patch by Hiroyuki Tanaka. Thanks!!!