Upgrade dotenv and dotenv-expand to latest version

[ericrallen]

The 5.*.* release of dotenv-expand fixes a bug where ENV variables set in the CLI as empty were getting overwritten by values in .env files, this bug was previously fixed in dotenv, but the fix was not ported over to dotenv-expand until recently.

It also adds better support for ENV variable values with $s in them and better handling of escaped $s in values.

I went ahead and updated dotenv to the latest version, as well. I believe the major version bump was just to drop support for older versions of Node.

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[iansu]

It's not really clear what the breaking changes are in dotenv-expand between version 4 and 5. @motdotla can you provide any details?

[ericrallen]

@iansu I believe the 4.2.0 -> 5.0.0 bump was the fix that prevented overwriting falsey values passed via the command line.

The 5.0.0 -> 5.1.0 bump was the better handling of ENV values with $s in them.

[ianschmitz]

Looks like this was already done in another PR. Thanks :)