Fixing bug when custom 'theme', 'custom_theme_widget' and 'ajax' options are used together

When using ajax: true and custom theming, the current loaded javascript looks for "dynamic_recaptcha" element id instead of the defined "custom_theme_widget" id. Moreover, when using custom theming, the "dynamic_recaptcha" div which contains the widget should be not created.

Code to reproduce the issue:

    <%= recaptcha_tags :display => {:theme => 'custom', :custom_theme_widget => 'recaptcha_widget'}, ajax => true %>