synchronize 4.1 release notes with CHANGELOGS. [ci skip]

/cc @chancancode
docs, Associations also raise on name conflicts. [ci skip]

Follow up to https://github.com/rails/rails/commit/580f0b61dc99c6854fa930a761d28a3ab08163f7#commitcomment-5293470
Add config to disable schema dump after migration

* Add a config on Active Record named `dump_schema_after_migration`
* Schema dump doesn't happen if the config is set to false
* Set default value of the config to true
* Set config in generated production environment file to false
* Update configuration guide
* Update CHANGELOG
Merge branch 'dump-schema-after-migration-flag' of git://github.com/emilsoman/rails into emilsoman-dump-schema-after-migration-flag

Conflicts:
	activerecord/CHANGELOG.md