Bump cryptography from 2.7 to 2.8

Bumps [cryptography](https://github.com/pyca/cryptography) from 2.7 to 2.8.
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst)
- [Commits](pyca/cryptography@2.7...2.8)

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

requirements.txt

@@ -2,7 +2,7 @@ amqp==2.5.2
 boto==2.49.0
 celery==4.3.0
 # click==7.0
-cryptography==2.7
+cryptography==2.8
 dj-database-url==0.5.0
 django==2.1.11
 django-celery-beat==1.5.0