Bump urllib3 from 1.22 to 1.24.2 in /requirements

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.22 to 1.24.2.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/master/CHANGES.rst)
- [Commits](urllib3/urllib3@1.22...1.24.2)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements/dev.txt

@@ -8,4 +8,4 @@ idna==2.5
 Pygments==2.2.0
 requests==2.18.2
 selenium==3.4.3
-urllib3==1.22
+urllib3==1.24.2