Bump certifi from 2018.11.29 to 2019.3.9

Bumps [certifi](https://github.com/certifi/python-certifi) from 2018.11.29 to 2019.3.9.
- [Release notes](https://github.com/certifi/python-certifi/releases)
- [Commits](certifi/python-certifi@2018.11.29...2019.03.09)

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

requirements.txt

@@ -1,5 +1,5 @@
 bcrypt==3.1.6
-certifi==2018.11.29
+certifi==2019.3.9
 cffi==1.12.2
 chardet==3.0.4
 colander==1.7.0