Update django-crispy-forms to 1.8.1

[pyup-bot]

This PR updates django-crispy-forms from 1.7.2 to 1.8.1.

Changelog

1.8.1

* Fixing FileField UI bug introduced with 1.8.0
* Remove is-valid css class for radio (bug introduced with 1.8.0)
* Various alignment and margin fixes for Bootstrap 4 template pack
* Documentation : using read-the-docs template, documenting how to run test suite, documenting use_custom_control help attributes

See the [1.8.1 Milestone](https://github.com/django-crispy-forms/django-crispy-forms/milestone/8?closed=1) for the full change list.

1.8.0

* Updated compatibility for Django 1.11, 2.1, 2.2 and 3.0 in line with Django's
supported versions policy.
* Numerous improvements to the Bootstrap 4 template pack, which may now be
considered mature.

Here are some changes that might affect custom templating you already have in
place when using Bootstrap 4 :

 * Using [Custom Forms for radio and checkbox](https://www.w3schools.com/bootstrap4/bootstrap_forms_custom.asp).
   You may disable this by setting  `FormHelper.use_custom_control = False`.
 * Using [Bootstrap 4 `"form-row"` class in place of `"row"`](https://getbootstrap.com/docs/4.3/components/forms/form-row).
 * Fixing [layout hierarchy between input and its label for checkbox and radio](https://getbootstrap.com/docs/4.0/components/forms/checkboxes-and-radios).

<small>Links are to the relevant Bootstap 4 docs.</small>

See the [1.8.0 Milestone](https://github.com/django-crispy-forms/django-crispy-forms/milestone/7?closed=1) for the full change list.

Many thanks to all involved in bringing together a great release!

Links

• PyPI: https://pypi.org/project/django-crispy-forms
• Changelog: https://pyup.io/changelogs/django-crispy-forms/
• Repo: https://github.com/django-crispy-forms/django-crispy-forms

[pyup-bot]

Closing this in favor of #274