Update newrelic.ini config to use heroku config vars

Using https://devcenter.heroku.com/articles/newrelic#node-js-agent-on-heroku as a guide, this seems like the thing to do.

Additional new relic should be put in code and should use the extra.heroku.php-includes option in composer.json
Added some best practices from the H5BP server configs
Added a default charset encoding
Merge pull request #72 from RobinvdVleuten/development

H5BP server config best practices
Update gems
Update gems
Add extension apcu
Use apcu from separate bundle
Merge branch 'development'
Update changelog for v0.3.1
Don't bundle APCu anymore, it lives in its own bundle now
Update default PHP to 5.5.8
Use NGINX 1.4.4
Add v0.3.2 to changelog
Enable pipefail again
Merge branch 'development'
Ignore failed extension downloads
Send Stderr to /dev/null for now
Add v0.3.3
Merge branch 'development'