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
Fix comment
Fix test
Merge pull request #7 from yui-knk/fix/comment

Fix comment of respond_with_resource_options
Merge pull request #8 from yui-knk/fix/test

Fix test of fields parameter
Test all versions of rails supported
Add dependency of responders
Dummy application supports Rails4.x
Garage::OptionalResponseBodyResponder#api_behavior support Rails 4.2
Drop support of Rails 3.x
Use Rails::VERSION::{MAJOR,MINOR}