Update README.md
Merge pull request #211 from just3ws/master

Vagrant upgrade with a little bit of email noise [ci skip]
Update postinstall.sh
Bumped VirtualBox/Vagrant versions in docs
Changed the box download link to use the V2 image
Fixed a missing configuration setting when the vagrant.yml is missing
Added a check to see if the vagrant.yml is present before starting Vagrant.
[skip ci] Changed the Stripe secret key in .env.example. It wasn't actually a secret value but looked real enough to be possibly confusing.
Added link to YouTube videos