Refactor for Capistrano 3

These changes update supply_drop to work with Capistrano 3 (as it is today). 

Some notable omissions:
- I had to remove the various writers, and the streaming and file output they support. Too much changed with the logging in Cap 3 for that model to make sense. This could be re-thought and re-imagined, and added back in. 

- Cap 3 also doesn't have transactions, so I implemented the lock/unlock semantics with `begin` ... `ensure` blocks. Hopefully that satisfies the expected contract of the transaction DSL for these purposes. 

Otherwise, the output is a little uglier, but it works fine for me on an Amazon Linux EC2 instance (RHEL/Centos compat). I haven't tried w/ Ubuntu yet, but I imagine it will be fine. I'd run it through the vagrant box tests before accepting the PR. 

Unit tests all pass.