use the dropbox provided content hash for flysystem checksum

[Gummibeer]

closes #91

[freekmurze]

Thanks!

It seems the tests are failing. Could you take a look at that?

[Gummibeer]

It's the lowest tests failing as the ChecksumProvider interface was added in v3.7 but the package supports thephpleague/flysystem: "^2.0.4 || ^3.0.0", - should the minimum version be bumped? Unsure how else we could fix the problem?
And two tests have a warning.

[freekmurze]

Let's bump the minimum versions indeed 👍

[Gummibeer]

@freekmurze done and also fixed the deprecation warning. I assume that this would lead to a new major version of the package? 🤔
As it's BC in several ways.

[freekmurze]

Thanks, I'll tag a new major version indeed.