Bump certifi from 2019.6.16 to 2019.9.11 in /requirements (#5498)

Bumps [certifi](https://github.com/certifi/python-certifi) from 2019.6.16 to 2019.9.11.
- [Release notes](https://github.com/certifi/python-certifi/releases)
- [Commits](https://github.com/certifi/python-certifi/compare/2019.06.16...2019.09.11)

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

requirements/common.txt

@@ -219,9 +219,9 @@ urllib3==1.25.3 \
     --hash=sha256:b246607a25ac80bedac05c6f282e3cdaf3afb65420fd024ac94435cabe6e18d1 \
     --hash=sha256:dbe59173209418ae49d485b87d1681aefa36252ee85884c31346debd19463232
 
-certifi==2019.6.16 \
-    --hash=sha256:046832c04d4e752f37383b628bc601a7ea7211496b4638f6514d0e5b9acc4939 \
-    --hash=sha256:945e3ba63a0b9f577b1395204e13c3a231f9bc0223888be653286534e5873695
+certifi==2019.9.11 \
+    --hash=sha256:fd7c7c74727ddcf00e9acd26bba8da604ffec95bf1c2144e67aff7a8b50e6cef \
+    --hash=sha256:e4f3620cfea4f83eedc95b24abd9cd56f3c4b146dd0177e83a21b4eb49e21e50
 
 # required by requests
 chardet==3.0.4 \