Support Django 2.1 by adding Widget.render's argument renderer

[tuky]

To support Django < 1.11, the argument renderer is not handed over to the super class, if it is None.

[erdem]

@tuky Thank you for your contribution and sorry for delayed reply, I was on holiday.

Did you test the widget with these changes on your project? Are all features worked?

I will re-check these changes in my first free time.

[tuky]

Welcome back. Yes, it's in use, but we are only using the GooglePointFieldWidget. My solution works, because the new base classes have renderer=None as an optional keyword argument.

[joshsweaney]

Used this on my projects and it is working fine.

[erdem]

@tuky @joshsweaney Thank you for the update guys! It merged.

[jckw]

Any chance you could update the PyPI package too? Looks like it hasn't had this update yet (last updated 5th June 2017).

[erdem]

@jckw I will push the new changes to PyPI in this week after solved this #77 (comment).

Thanks.