Change environment variables to use getenv

[tractorcow]

Replaces #28

[ichaber]

Thanks for implementing the necessary changes @tractorcow 🌟

[chillu]

@tractorcow There's some PHPCS failures, could you have a look? Charlie is approving the changes (he's sitting next to me), so feel free to self merge - unless you think it needs someone else's review. This is critical for SSP 4.x operation, high priority

[tractorcow]

Ok fixed. Also moved to vendormodule.

[sminnee]

Does this module expose front-end resources? If not, I don't think this module is a requirement.

Also, are we expecting that modules are allowed to rely on framework supplying this as a requirement, or do we expect that every module maintainer has to add this line? I'd suggest that relying on framework to properly take care of the implications of using silverstripe-vendormodule would be an acceptable division of responsibility.

[tractorcow]

Ah good point, removed.

Modules without exposed resources don't need the plugin.

[chillu]

I've also published a 4.0.1 release: https://github.com/silverstripe/silverstripe-dynamodb/releases/tag/4.0.1, and adjusted the branch alias to match the main release line: #31