use default new relic environment variables

Untested, should be working change that will likely require small changes to your upstream configuration (replace NEW_RELIC_INI with NEW_RELIC_CONFIG_FILE, replace NEW_RELIC_STAGE with NEW_RELIC_ENVIRONMENT). Change attached to #567 
Can you add there both NEW_RELIC_LICENSE_KEY and NEW_RELIC_CONFIG_FILE (with default '')?
It makes it easier to know what env vars we are using overall.