Update py-flask-frozen to 0.15.

Version 0.15
~~~~~~~~~~~~

Released on 2017-06-27.

* Add ``Freezer.freeze_yield()`` method to make progress reporting easier.
  (Thanks to Miro Hron��ok.)

Version 0.14
~~~~~~~~~~~~

Released on 2017-03-22.

* Add the ``FREEZER_SKIP_EXISTING`` configuration to skip generation
  of files already in the build directory. (Thanks to Antoine Goutenoir.)
* Add shared superclass ``FrozenFlaskWarning`` for all warnings.
  (Thanks to Miro Hron��ok.)

(kleink)