Orbital: Fix void method signature

Gateway#void should have the method signature:

    def void(identifier, options = {})

But Orbital had the signature:

    def void(amount, identifier, options = {})

This fixes the signature, with a deprecation warning if the old
signature is called.

Also passes the amount (if passed via options) on to Orbital, which
allows for partial voids.

Closes #531.
Orbital: Remove the remote secondary url test

The fallback to the secondary url is already tested in the unit tests,
and the remote test for it was completely broken.
Orbital: Fix warnings in remote test
Orbital: Add void unit test

This adds a unit test for void, and also moves the deprecation test for
void from the remote test to the unit test.