Bump certifi from 2019.3.9 to 2019.6.16

Bumps [certifi](https://github.com/certifi/python-certifi) from 2019.3.9 to 2019.6.16.
- [Release notes](https://github.com/certifi/python-certifi/releases)
- [Commits](certifi/python-certifi@2019.03.09...2019.06.16)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -1,5 +1,5 @@
 bleach==3.1.0
-certifi==2019.3.9
+certifi==2019.6.16
 chardet==3.0.4
 docutils==0.14
 idna==2.8