Bump validators from 0.12.4 to 0.12.6 in /requirements

Bumps [validators](https://github.com/kvesteri/validators) from 0.12.4 to 0.12.6.
- [Release notes](https://github.com/kvesteri/validators/releases)
- [Changelog](https://github.com/kvesteri/validators/blob/master/CHANGES.rst)
- [Commits](https://github.com/kvesteri/validators/commits)

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

Author: dependabot[bot]

requirements/common.txt

@@ -50,5 +50,5 @@ six==1.12.0
 speaklater==1.3
 twilio==6.32.0
 unittest2==1.1.0
-validators==0.12.4
+validators==0.12.6
 webassets==0.12.1