bonding: onlyVerifier modifier for slashTranscoder()

[kyriediculous]

What does this pull request do? Explain your changes. (required) This PR replaces onlyTicketBroker with a newly introduced onlyVerifier modifier

Specific updates (required)

• added an onlyVerifier modifier that checks if the caller is the Verifier contract (0x000...) in streamflow
• replace onlyTicketBroker with onlyVerifier on bondingManager.slashTranscoder()
• adjust slashTranscoder unit tests in BondingManager.js to call from the mock verifier instead of ticketBroker

How did you test each of these updates (required) Adjusted & ran unit and integration tests

Does this pull request close any open issues? Fixes #315

Checklist:

• [ ] README and other documentation updated
• [x] All unit & integration tests pass