Update jest-junit integration

[simonplend]

testResultsProcessor support is deprecated. Please use jest reporter. See https://github.com/jest-community/jest-junit#usage:

{
  "reporters": [
    "jest-junit"
  ]
}

First seen: https://circleci.com/gh/Financial-Times/n-test/490

Can reproduce locally by running:

CI=true make unit-test

Instructions for updating here: https://github.com/jest-community/jest-junit#usage