Fix dropped widget attributes with Django >=1.11

[bentrm]

The signature of the private Widget.build_attrs method has changed with Django 1.11. Due to this attributes added to a TinyMCE widget by other modules will be dropped. I was realizing this when using this module in conjunction with the django-modeltranslation package. The added method is a combination of the pre-1.11 and 1.11 signature. It is overridden to keep compatibility with earlier versions of Django.

django-modeltranslation uses css classes to group translated fields into tabs. Theses classes have been lost if used with a TinyMCE widget.

Before changes:

After changes:

[romanvm]

Thanks, but please submit the PR against the develop branch.

[bentrm]

There you go.

[romanvm]

Thanks!