What are Environment Variables and why are they useful?

[nelsonic]

See:

• https://en.wikipedia.org/wiki/Environment_variable
• http://stackoverflow.com/questions/9198310/how-to-set-node-env-to-production-development-in-os-x
• http://docs.travis-ci.com/user/environment-variables/

gem install travis
cd my_project
travis encrypt MY_SECRET_ENV=super_secret --add env.matrix