Remove unused code validator subclass

This is just an unneeded specialisation of
ParameterMustBeOneOfTheseValidator.
Merge pull request #354 from alphagov/feature/delete-unused-code

Remove unused code validator subclass
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Upgrades requests to 2.4

Seems to be a minor issue after they upgraded their vendored version
of urllib3.

https://github.com/kennethreitz/requests/issues/2192
Merge pull request #355 from alphagov/feature/dependency-upgrades

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