dotenv dependency missing + using deprecated alias load

[doramatadora]

This package requires dotenv. Some teams (Internal Products, Reliability Engineering) do not have dotenv in their shared tooling. Example of failing CI build / error.

The current implementation also requires that dotenv be <7, because it uses the deprecated alias 'load': https://github.com/Financial-Times/fastly-tools/blob/master/tasks/deploy.js#L23 (instead of config).

See error for ^7 & the dotenv changelog: https://github.com/motdotla/dotenv/blob/master/CHANGELOG.md#700---2019-03-12