Fix signature of Bootstrap.__init__() incompatible with older version.

[davidlesieur]

The Bootstrap class is now a subclass of Bootstrap4, and provides a deprecation warning. That should prevent existing code from breaking. However, this purpose is defeated by a change in the signature of Bootstrap.__init__() that can break existing code.

This change restores the previous signature.

[davidlesieur]

This matters for existing code that relies on the ability to pass the app object to the init_app() method instead of __init__().

[greyli]

Good catch, thanks!