Update CircleCI to exercise all dependencies on every commit

[AlexSc]

As a CTO
I want to update dependencies fearlessly
Therefore I need to be able to trigger the SDK build pipeline at any time in order to exercise the build pipeline code, without actually pushing a new release of the SDK

Cloned from GoCarrot/teak-ios#97. See implementation plan comment there for approach — same pattern adapted for this repo's Rake build, UPM packaging, and Unity-specific dependencies.

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Repo: teak-unity

[AlexSc]

Dev builds should also push to a UPM `dev` branch so Unity devs can test via Package Manager before a tagged release.