Update ruby-amqp to 1.6.0.

## Changes Between 1.5.x and 1.6.0 (Apr 4th, 2016)

### amq-protocol Update

Minimum `amq-protocol` version is now `2.0.1`.

### Provide More Details in TCP Connection Failure Exception

Contributed by Neil Hooey.

GH issue: [#222](https://github.com/ruby-amqp/amqp/issues/222).

### Ensures frameset is cleared after an unhandled exception

Ensures frameset is cleared after an unhandled exception.
This avoids confusing exceptions such as

```
undefined method `method_class' for #<AMQ::Protocol::BodyFrame:0x0000001e8a60b0>
```

Contributed by Michael Lutsiuk.

GH issue: [#218](https://github.com/ruby-amqp/amqp/issues/218)

(taca)