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>

requirements.txt

@@ -1,7 +1,7 @@
 algoliasearch==2.0.4
 attrs==19.1.0
 bcrypt==3.1.6
-certifi==2019.3.9
+certifi==2019.6.16
 cffi==1.12.3
 chardet==3.0.4
 colander==1.7.0