Add twitter test configuration

Setting of Twitter authentication key is leaking only test.

$ bundle exec rake
rake aborted!
Settingslogic::MissingSetting: Missing setting 'twitter' in /home/travis/build/328--/eXtick/config/settings.yml
/home/travis/build/328--/eXtick/vendor/bundle/ruby/2.1.0/gems/settingslogic-2.0.9/lib/settingslogic.rb:189:in `missing_key'
/home/travis/build/328--/eXtick/vendor/bundle/ruby/2.1.0/gems/settingslogic-2.0.9/lib/settingslogic.rb:117:in `method_missing'
/home/travis/build/328--/eXtick/vendor/bundle/ruby/2.1.0/gems/settingslogic-2.0.9/lib/settingslogic.rb:66:in `method_missing'
/home/travis/build/328--/eXtick/config/initializers/omniauth.rb:2:in `block in <top (required)>'
：
Tasks: TOP => test:run => test:units
(See full trace by running task with --trace)
The command "bundle exec rake" exited with 1.

↓

$ bundle exec rake

Finished in 0.978678s, 8.1743 runs/s, 14.3050 assertions/s.

8 runs, 14 assertions, 0 failures, 0 errors, 0 skips