Remove orphaned dependencies

[confused-Techie]

While attempting to find where we still used emissary here (to attempt to remove es5-ext), I discovered that we actually don't.

Even more there was a surprising amount of dependencies in this repo that we just flat out don't use.

[savetheclocktower]

Just following up with some archaeology.

First the devDependencies:

• cpy-cli used to be part of the build process.
• json-diff was used in the specs optionally — there were some lines you could uncomment for more debugging information if a spec failed.
• standard was used for linting instead of ESLint. (I imagine we got rid of linting when we decaffeinated this package, which is fine. We can reinstate it later if we want to.)

And now the dependencies:

• emissary was once used for events instead of event-kit. I imagine it just didn't get removed when Atom migrated to event-kit.
• serializable was once used for… well, serialization. I can only guess that the move to superstring removed most references to serializable.

At any rate: these definitely aren't used anymore, so they're safe to remove even if we didn't understand why they were added in the first place!