Removed the bucket -b option.

The option -b is also used by Puma to bind to a TCP/Unix socket. To work around
this we'll only support --bucket instead of both -b/--bucket.
Release 2.1.2.

This release removes the -b option in favour of only supporting --bucket. While
this breaks semantic versioning the -b option was never supposed to be added in
the first place (meaning it's a bug). For this I'm willing to break semantic
versioning.