Check that missing node is not null

[alexislefebvre]

I accidentaly wrote this in a test:

    And the JSON node "root[0].createdAt" should not be null
    And the JSON node "root[0].createdAt" should not exist

The expected behaviour is that the node doesn't exist, but should not be null didn't failed.

So I add this test to check if it will fail or not on CI.

[alexislefebvre]

Travis CI didn't report result to the PR. And the CI is broken now: https://travis-ci.org/github/Behatch/contexts/jobs/723377887