Change requirements to Wagtail>=2.15

[Alexandre Leray]

Hi,

using Wagtail 2.13 I get an error:

  File "/home/foo/.local/share/virtualenvs/app-4PlAip0Q/lib/python3.7/site-packages/wagtail_draftail_snippet/urls.py", line 23, in <module>
    "choose/", chooser.ChooseView.as_view(), name="choose_generic"
AttributeError: module 'wagtail.snippets.views.chooser' has no attribute 'ChooseView'

Seems like wagtail.snippets.views.chooser.ChooseView was added in Wagtail 2.15.

https://github.com/wagtail/wagtail/blob/v2.15/wagtail/snippets/views/chooser.py#L77