Merge okcomputer-newrelic-ignore gem into the core

Merge the functionality of https://github.com/sportngin/okcomputer-newrelic-ignore into the OkComputer::Engine initializer.

Reasons:
1) There's no practical case where a NewRelic user would want NewRelic to count OkComputer.
2) okcomputer-newrelic-ignore is a Monkey Patch, which is anti-pattern and is brittle if OkComputer changes it's structure.
3) My app has enough gems already, would rather not add another one that contains < 10 lines of app code.