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