Bump freezegun from 0.3.14 to 0.3.15

Bumps [freezegun](https://github.com/spulec/freezegun) from 0.3.14 to 0.3.15.
- [Release notes](https://github.com/spulec/freezegun/releases)
- [Changelog](https://github.com/spulec/freezegun/blob/master/CHANGELOG)
- [Commits](spulec/freezegun@0.3.14...0.3.15)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -2,5 +2,5 @@ coverage==5.0.3
 Django==1.11.28 # pyup: <2.0
 django-debug-toolbar==2.2
 docutils==0.16
-freezegun==0.3.14
+freezegun==0.3.15
 django-formtools==2.2