Remove package.json lint rule for no devdeps
We are using npm-package-json-lint to check that our package.json files comply with certain rules.
One of these rules is configured so that you are not allowed to use devDependencies in the packages, but it's OK at the top-level. However this test fails because we have devDependencies in the eslint config package.
Seems to me that we should just disable this rule. I'm not sure why it has ever passed TBH.
This is because the devDependencies need to be moved from the ESLint package to the root package.json
This just hasn't happened yet and is why this rule is only set to warning which is why CI runs currently pass
https://github.com/humanmade/coding-standards/blob/9aedd9d07d2575a694f9b13d0d1e0be410ae8274/package.json#L38
E.g. via https://app.travis-ci.com/github/humanmade/coding-standards/jobs/578616221#L262-L270