chore(deps): Bump pillow from 5.1.0 to 6.2.0 in /backend (#12)

Bumps [pillow](https://github.com/python-pillow/Pillow) from 5.1.0 to 6.2.0.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](python-pillow/Pillow@5.1.0...6.2.0)

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

backend/requirements.txt

@@ -2,7 +2,7 @@ dj-database-url==0.5.0
 Django==2.0.12
 django-cors-headers==2.2.0
 djangorestframework==3.9.1
-Pillow==5.1.0
+Pillow==6.2.0
 PyJWT==1.6.1
 python-decouple==3.1
 pytz==2018.3