Bump requests from 2.18.1 to 2.20.0 in /elastalert

Bumps [requests](https://github.com/requests/requests) from 2.18.1 to 2.20.0.
- [Release notes](https://github.com/requests/requests/releases)
- [Changelog](https://github.com/psf/requests/blob/master/HISTORY.md)
- [Commits](psf/requests@v2.18.1...v2.20.0)

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

elastalert/requirements.txt

@@ -30,7 +30,7 @@ pytest==3.1.2
 python-dateutil==2.6.0
 pytz==2017.2
 PyYAML==5.1
-requests==2.18.1
+requests==2.20.0
 requests-mock==1.3.0
 requests-oauthlib==0.8.0
 requests-toolbelt==0.8.0