Switch env variables prefix from REACT_APP to GATSBY

[leopuleo]

Hello,
first of all, thanks for this amazing starter!
I experienced some issues by deploying it via CI/CD tools like CircleCI:

since the environment variables are stored inside the platform, and not in a .env file, Gatsby build process fails.

I solved it by changing the prefix from REACT_APP to GATSBY, as suggested here:

https://www.gatsbyjs.org/docs/environment-variables/#client-side-javascript

I guess this happens because Firebase is a client-side script.

Let me know what you think!
Cheers,

Leonardo

[rwieruch]

Thank you for your help here! Another PR fixed it as well, so I have merged to from there #47