Bump cffi from 1.11.5 to 1.12.0

Bumps [cffi](https://bitbucket.org/cffi/release-doc) from 1.11.5 to 1.12.0.
- [Commits](https://bitbucket.org/cffi/release-doc/commits)

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

requirements.txt

@@ -1,6 +1,6 @@
 bcrypt==3.1.6
 certifi==2018.11.29
-cffi==1.11.5
+cffi==1.12.0
 chardet==3.0.4
 colander==1.7.0
 colorama==0.4.1