Add set_deref_options call to Promise constructor

[rkday]

Fixes unintitialised variable warnings, and matches delay.rb.

I spotted these warnings while (I think) waiting for a Promise to be fulfilled:

/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1/lib/concurrent/dereferenceable.rb:101: warning: instance variable @do_nothing_on_deref not initialized
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1/lib/concurrent/dereferenceable.rb:102: warning: instance variable @copy_on_deref not initialized
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1/lib/concurrent/dereferenceable.rb:103: warning: instance variable @dup_on_deref not initialized
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1/lib/concurrent/dereferenceable.rb:104: warning: instance variable @freeze_on_deref not initialized

and saw that promise.rb was calling apply_deref_options (and documenting the relevant constructor values) but not set_deref_options.

[coveralls]

Coverage decreased (-0.08%) when pulling 9d588f3 on rkday:promise_deref into 6af1316 on ruby-concurrency:master.

[jdantonio]

Thank you!