hash/conversions.rb needs active_support/core_ext/string/inflections because it uses singularize and a few others

requires active_support/inflector/methods active_support/core_ext/string/inflections in local_cache.rb because it uses underscore

requires active_support/core_ext/array/extract_options in mem_cache_store.rb because it uses extract_options!

[#3636 state:committed]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

[#3641 state:committed]
Signed-off-by: NJeremy Kemper <jeremy@bitsweat.net>

Rename Model.engine to active_relation_engine. Cache arel_table and the method takes no arguments now

Put MySQL and Postgres in an :only block so you can gem bundle --only default to leave out MySQL and Postgres