Fix up warnings

When running my ruby-diameter test suite (which uses concurrent-ruby and puts Ruby in debug mode), I get a ton of warnings:

```
$ rake test
/home/rkd/.rvm/rubies/ruby-2.1.5/bin/ruby -w -I"lib:test" -I"/home/rkd/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0" "/home/rkd/.rvm/rubies/ruby-2.1.5/lib/ruby/2.1.0/rake/rake_test_loader.rb" "test/test_avp.rb" "test/test_message_parsing.rb" "test/test_stack.rb" 
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/simplecov-html-0.8.0/lib/simplecov-html.rb:58: warning: possibly useless use of a variable in void context
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/atomic/atomic_fixnum.rb:28: warning: `-' after local variable or literal is interpreted as binary operator
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/atomic/atomic_fixnum.rb:28: warning: even though it seems like unary operator
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/atomic/atomic_fixnum.rb:29: warning: `-' after local variable or literal is interpreted as binary operator
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/atomic/atomic_fixnum.rb:29: warning: even though it seems like unary operator
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/observable.rb:49: warning: `*' interpreted as argument prefix
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/executor/indirect_immediate_executor.rb:44: warning: private attribute?
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/executor/serialized_execution.rb:15: warning: `*' interpreted as argument prefix
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/lazy_register.rb:52: warning: shadowing outer local variable - h
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/async.rb:84: warning: shadowing outer local variable - args
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/promise.rb:103: warning: `&' interpreted as argument prefix
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/scheduled_task.rb:29: warning: `&' interpreted as argument prefix
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/timer_task.rb:320: warning: assigned but unused variable - success
/home/rkd/rb_diameter/lib/diameter/stack.rb:229: warning: `&' interpreted as argument prefix
/home/rkd/rb_diameter/lib/diameter/stack.rb:230: warning: `&' interpreted as argument prefix
Run options: --seed 50108

# Running tests:

.................../home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/executor/ruby_thread_pool_worker.rb:48: warning: instance variable @thread not initialized
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/executor/ruby_thread_pool_worker.rb:48: warning: instance variable @thread not initialized
./home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/executor/ruby_thread_pool_worker.rb:48: warning: instance variable @thread not initialized
/home/rkd/.rvm/gems/ruby-2.1.5@diameter/gems/concurrent-ruby-0.7.1-x86_64-linux/lib/concurrent/executor/ruby_thread_pool_worker.rb:48: warning: instance variable @thread not initialized
.......

Finished tests in 0.011183s, 2414.4031 tests/s, 7064.3648 assertions/s.

27 tests, 79 assertions, 0 failures, 0 errors, 0 skips
```

This fixes all of those up. (It also fixes a typo that I missed in https://github.com/ruby-concurrency/concurrent-ruby/pull/203).