`Flux.Optimise.update!` no longer works on `Scheduler`

[lassepe]

Since the latest release, Flux.Optimise.update! no longer works on a Scheduler because the type of the generic optimization has been narrowed to `AbstractOptimiser in the latest release of Flux.

As an aside: it may be worth adding some basic integration tests to this repo to test the interoperability of ParameterSchedulers with Flux.

[darsnack]

it may be worth adding some basic integration tests

As the merging of #29 indicates...I thought these tests existed which is an oversight on my part.