Remove use of the old contributors.json.

[pelson]

This PR makes the CLA checker only look at the new CLA, rather than the old contributors.json document. Anybody who has not signed the new CLA will now see failures in PRs, which can easily be solved by signing the document (https://scitools.org.uk/cla/v4/form).

This will speed along issues such as SciTools/cf-units#143 which are to be repeated throughout the SciTools org.

Agreement from one of @ajdawson or @bjlittle required to merge.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}