Bump colander from 1.5.1 to 1.6.0

Bumps [colander](https://github.com/Pylons/colander) from 1.5.1 to 1.6.0.
- [Release notes](https://github.com/Pylons/colander/releases)
- [Changelog](https://github.com/Pylons/colander/blob/master/CHANGES.rst)
- [Commits](https://github.com/Pylons/colander/commits)

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

requirements.txt

@@ -3,7 +3,7 @@ bcrypt==3.1.6
 certifi==2018.11.29
 cffi==1.11.5
 chardet==3.0.4
-colander==1.5.1
+colander==1.6.0
 colorama==0.4.1
 cornice==3.5.1
 cornice-swagger==0.7.0