PoolManager resumes a task from inside a task

As of 0.15.0 I am seeing this error "Cannot resume a task from inside of a task" when using a pool.  I was able to repro with this little snippet.  Running JRuby 1.7.2.

``` ruby
class Worker
  include Celluloid
  def perform(callable, *args)
    callable.call(*args)
  end
end

callable = lambda { |number| number + 1 }

pool = Worker.pool(:size => 2)
10.times { pool.async.perform(callable, 1) }
```

```
jruby-1.7.2 :057 > Celluloid::PoolManager crashed!
RuntimeError: Cannot resume a task from inside of a task
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/tasks.rb:96:in `resume'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/responses.rb:11:in `dispatch'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/actor.rb:327:in `handle_message'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/pool_manager.rb:97:in `__provision_worker__'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/pool_manager.rb:38:in `_send_'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/pool_manager.rb:122:in `method_missing'
org/jruby/RubyKernel.java:1806:in `public_send'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/calls.rb:25:in `dispatch'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/calls.rb:122:in `dispatch'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/actor.rb:322:in `handle_message'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/actor.rb:416:in `task'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/tasks.rb:54:in `initialize'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/tasks.rb:46:in `initialize'
/vendor/jruby/1.9/gems/celluloid-0.15.0/lib/celluloid/tasks/task_fiber.rb:13:in `create'