Update ruby-celluloid to 0.17.3.

0.17.3 (2016-01-18)
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* [#701](https://github.com/celluloid/celluloid/pull/701)
  Conditions in loose threads loop does not take into account the difference between
  backtraces from ruby 2.0.0 and greater than. Fixes celluloid/celluloid-io#165.
  ([@TiagoCardoso1983])

* [#700](https://github.com/celluloid/celluloid/pull/700)
  Set celluloid logger level to info by default unless debug is enabled. Fixes #667.
  ([@ioquatix])

* [#695](https://github.com/celluloid/celluloid/pull/695)
  Extending the condition event handler with the block; this solves a bug
  introduced in jruby >9.0.0.0, which breaks with an ArgumentError exception,
  apparently due to the way to_proc procs are passed arguments. Fixes #694.
  ([@TiagoCardoso1983])

* [#689](https://github.com/celluloid/celluloid/pull/689)
  Simplified sync, async and future proxies by providing specific AbstractCall base.
  ([@ioquatix])

* [#688](https://github.com/celluloid/celluloid/pull/688)
  Fix failure to remove dead actors from sets, e.g. celluloid-supervision.
  ([@ioquatix])

* [#686](https://github.com/celluloid/celluloid/pull/686)
  Print out method name to help debugging method call which caused dead actor error.
  ([@ioquatix])

* [#682](https://github.com/celluloid/celluloid/pull/682)
  Remove excess call/block require.

* [#666](https://github.com/celluloid/celluloid/pull/666)
  Don't catch IOError.

(taca)