fix travis links in readme

links were redirects
pass over the code comments

* Highlights the requirement of an attributes method.
* Removes some details that depend on the implementation of the class including the module.
* Applies guidelines here and there.
fixes a test, and explains why AR::AttributeMethods checks defined?(@attributes) in some places
Making the retrieval of the cache store class a method, also wrote
comments for the expand_cache_key method.
Use new hash syntax in module delegation docs [ci skip]
Fix typo in serialized_attribute_test. [ci skip]
Merge pull request #10368 from demands/fix-typo

Fix typo in serialized_attribute_test. [ci skip]
Merge pull request #10361 from teeparham/patch-1

Fix travis links in readme [ci skip]
Merge pull request #10364 from wangjohn/refactoring_activesupport_cache

Making the retrieval of the cache store class a method
Delegate #unscope query method
StatementInvalid takes WrappedDatabaseException's place
Merge pull request #10372 from jeremy/statement-invalid-wraps-underlying-exception

StatementInvalid takes WrappedDatabaseException's place
Bump activerecord-deprecated_finders. Fixes #10304