Suppress warnings

[tagomoris]

These changes are to suppress warnings of ruby interpreter.

[tagomoris]

Currently, there are only warnings about ServerEngine "instance variable @file_dev not initialized" once tests started to run.
And that warning should be removed if this pull-request will be merged: treasure-data/serverengine#47

[tagomoris]

There is a problem that Ctrl-c cannot stop whole test runner if t.warning = true is set.
Shell shows prompt, but test runner continues to run.
Does someone know about this?

[tagomoris]

After rebase with serverengine 1.6.4, there are no warnings after running tests.
There are still 3 warning from Fluentd's code and many from cool.io, but I think it's ok to enable verbose warning in default.

[tagomoris]

This result shows how many warnings are left:
https://travis-ci.org/fluent/fluentd/jobs/131582636

@repeatedly There are many warnings from cool.io. Please take care when you have time :)

/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/loop.rb:41: warning: method redefined; discarding old initialize
(eval):1: warning: method redefined; discarding old on_read
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/io.rb:79: warning: previous definition of on_read was here
(eval):1: warning: method redefined; discarding old on_write_complete
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/io.rb:83: warning: previous definition of on_write_complete was here
(eval):1: warning: method redefined; discarding old on_close
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/io.rb:87: warning: previous definition of on_close was here
(eval):1: warning: method redefined; discarding old on_readable
(eval):1: warning: method redefined; discarding old on_writable
(eval):1: warning: method redefined; discarding old on_timer
(eval):1: warning: method redefined; discarding old on_signal
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/async_watcher.rb:24: warning: previous definition of on_signal was here
(eval):1: warning: method redefined; discarding old on_connection
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/listener.rb:35: warning: previous definition of on_connection was here
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/dns_resolver.rb:171: warning: assigned but unused variable - ancount
(eval):1: warning: method redefined; discarding old on_success
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/dns_resolver.rb:89: warning: previous definition of on_success was here
(eval):1: warning: method redefined; discarding old on_failure
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/dns_resolver.rb:93: warning: previous definition of on_failure was here
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/socket.rb:30: warning: method redefined; discarding old attach
(eval):1: warning: previous definition of attach was here
(eval):1: warning: method redefined; discarding old on_connect
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/socket.rb:42: warning: previous definition of on_connect was here
(eval):1: warning: method redefined; discarding old on_connect_failed
/home/travis/build/fluent/fluentd/vendor/bundle/ruby/2.3.0/gems/cool.io-1.4.3/lib/cool.io/socket.rb:46: warning: previous definition of on_connect_failed was here