Fixes license key bash command in README

Hi, jessicard.

Just a small fix to the README and newrelic.yml to use `NEW_RELIC_LICENSE_KEY`. Also removed an empty .gitignore that was in public/img/.

This is the best writeup I've seen on how to get New Relic to play nice with a Rack app. Still having trouble getting Availability Monitoring set up, though. Trying to access it from my control panel on New Relic's site keeps redirecting to their errors page. :-/ 

In any case, great example app!

Cheers,
O-I