Details:
Previously this hook required that you either be inside the vagrant
Zulip dev virtual machine when you ran git commit or that you had setup
your Zulip dev environment manually.
Now the script runs the linter via vagrant ssh if the following
conditions are met:
- VIRTUAL_ENV is not set
- vagrant is installed and a .vagrant directory exists in the repo
Otherwise the linter is run as it was before.
[tweaked to fix a few style things by tabbott]