JRuby: occasionally builds fail

Like this:

```
Failures:

1) Truck behaves like an active object
Failure/Error: Unable to find matching line from backtrace
NoMethodError:
undefined method `mine_state' for #
Shared Example Group: "an active object" called from ./spec/entities/truck_spec.rb:5
# ./lib/mining_depot/entities/mine.rb:88:in `machine'
# ./lib/mining_depot/entities/mine.rb:87:in `machine'

Deprecation Warnings:
```

and this:

```
Failures:

1) Mine#status return value
Failure/Error: Unable to find matching line from backtrace
NoMethodError:
undefined method `speed' for #
# ./lib/mining_depot/entities/mine.rb:98:in `machine'
# ./lib/mining_depot/entities/mine.rb:87:in `machine'
```