Don't require 'ac/responder' explicitly

We should require 'responder' with Rails4.2+ and responders gem in
Bundler requiring.
Add notice about responders gem
Merge pull request #4 from taiki45/responder-problems

Don't require 'ac/responder' explicitly
Setup for Travis CI
Test for Travis CI
Fix build script, and remove 1.9.3
Use group to enable rspec in test environment
Create db before executing spec
Fix typo
Merge pull request #5 from yui-knk/fix/typo

Fix typo
Merge branch 'master' into bypass_the_cache_in_case_of_not_saved_model