add travis.yml

Many test fails in the current master branch as followings (I used ruby-2.0.0-p195). Let's use travis so that we can notice it easily. 

    1) ServerEngine::Daemon run and graceful stop
     Failure/Error: test_state(:server_stop_graceful).should == 1
       expected: 1
            got: 0 (using ==)
     # ./spec/daemon_spec.rb:17:in `block (2 levels) in <top (required)>'
    
    2) ServerEngine::Daemon signals
     Failure/Error: test_state(:server_reload).should == 1
       expected: 1
            got: 0 (using ==)
     # ./spec/daemon_spec.rb:31:in `block (2 levels) in <top (required)>'
    
    3) ServerEngine::MultiWorkerServer scale up
     Failure/Error: test_state(:worker_run).should == 3
       expected: 3
            got: 4 (using ==)
     # ./spec/multi_process_server_spec.rb:21:in `block (3 levels) in <top (required)>'
    
    4) ServerEngine::MultiWorkerServer scale up
     Failure/Error: test_state(:worker_run).should == 3
       expected: 3
            got: 6 (using ==)
     # ./spec/multi_process_server_spec.rb:21:in `block (3 levels) in <top (required)>'
    
    5) ServerEngine::MultiWorkerServer scale down
     Failure/Error: test_state(:worker_stop).should == 2
       expected: 2
            got: 0 (using ==)
     # ./spec/multi_process_server_spec.rb:49:in `block (3 levels) in <top (required)>'
    
    6) ServerEngine::SignalThread call handler
     Failure/Error: n.should == 1
       expected: 1
            got: 0 (using ==)
     # ./spec/signal_thread_spec.rb:20:in `block (2 levels) in <top (required)>'
    
    7) ServerEngine::SignalThread signal in handler
     Failure/Error: n.should == 3
       expected: 3
            got: 0 (using ==)
     # ./spec/signal_thread_spec.rb:49:in `block (2 levels) in <top (required)>'
    
    8) ServerEngine::SignalThread stop in handler
     Failure/Error: t = SignalThread.new do |st|
     ThreadError:
       can't be called from trap context
     # ./lib/serverengine/signal_thread.rb:138:in `unlock'
     # ./lib/serverengine/signal_thread.rb:138:in `ensure in enqueue'
     # ./lib/serverengine/signal_thread.rb:138:in `enqueue'
     # ./lib/serverengine/signal_thread.rb:46:in `block in trap'
     # ./lib/serverengine/signal_thread.rb:26:in `call'
     # ./lib/serverengine/signal_thread.rb:26:in `new'
     # ./lib/serverengine/signal_thread.rb:26:in `initialize'
     # ./spec/signal_thread_spec.rb:55:in `new'
     # ./spec/signal_thread_spec.rb:55:in `block (2 levels) in <top (required)>'
    
    9) ServerEngine::SignalThread should not deadlock
     Failure/Error: t.join
     ThreadError:
       can't be called from trap context
     # ./lib/serverengine/signal_thread.rb:138:in `unlock'
     # ./lib/serverengine/signal_thread.rb:138:in `ensure in enqueue'
     # ./lib/serverengine/signal_thread.rb:138:in `enqueue'
     # ./lib/serverengine/signal_thread.rb:46:in `block in trap'
     # ./spec/signal_thread_spec.rb:78:in `call'
     # ./spec/signal_thread_spec.rb:78:in `join'
     # ./spec/signal_thread_spec.rb:78:in `block (3 levels) in <top (required)>'

     # ./spec/signal_thread_spec.rb:77:in `each'
     # ./spec/signal_thread_spec.rb:77:in `block (2 levels) in <top (required)>'
    
    10) ServerEngine::Supervisor start and graceful stop
     Failure/Error: test_state(:server_before_run).should == 1
       expected: 1
            got: 0 (using ==)
     # ./spec/supervisor_spec.rb:18:in `block (2 levels) in <top (required)>'
    
    11) ServerEngine::Supervisor immediate stop
     Failure/Error: test_state(:server_stop).should == 1
       expected: 1
            got: 0 (using ==)
     # ./spec/supervisor_spec.rb:43:in `block (2 levels) in <top (required)>'
    
    12) ServerEngine::Supervisor graceful restart
     Failure/Error: test_state(:server_stop).should == 1
       expected: 1
            got: 0 (using ==)
     # ./spec/supervisor_spec.rb:62:in `block (2 levels) in <top (required)>'
    
    13) ServerEngine::Supervisor immediate restart
     Failure/Error: test_state(:server_stop).should == 1
       expected: 1
            got: 0 (using ==)
     # ./spec/supervisor_spec.rb:83:in `block (2 levels) in <top (required)>'
    
    14) ServerEngine::Supervisor reload
     Failure/Error: test_state(:server_stop).should == 1
       expected: 1
            got: 0 (using ==)
     # ./spec/supervisor_spec.rb:104:in `block (2 levels) in <top (required)>'