Secure environment variables

[nelsonic]

Get Excited!

• Adds section on Environment Variables
• Adds link to a more detailed example (Learn TDD) which has a more practical example for Travis-CI
• Adds link to https://github.com/nelsonic/learn-environment-variables tutorial ...

[nelsonic]

@iteles please review and merge if happy.
Also we need to tweet the fact that we have updated our tutorial (finally)
My suggestion would be:

"Are you using Continuous Integration to Test the Build of your @nodejs projects?
We just updated our Tutorial on using @TravisCI so its easier than ever to get started!"

Or words to that effect.
and use the image:

https://raw.githubusercontent.com/dwyl/repo-badges/master/highresPNGs/build-passing.png

Thanks!

[nelsonic]

@iteles thanks for reviewing/merging. please paste link to tweet so we can re-tweet. thanks!

[iteles]

https://twitter.com/dwylhq/status/636973678358609920

[nelsonic]

Sweet. thanks! 🎉