added a visitor for Set objects
"Relational Database Managements systems", not "Relational Database Managements System systems"
Remove deprecated `Arel::Table.table_cache` with no replacement
Remove deprecated `Arel::Table#columns` with no replacement
Cache visitor dispatch on a per-visitor basis
Improve performance of #uniq across a large number of nodes
Merge pull request #206 from SamSaffron/master

Improve performance of #uniq across a large number of nodes
Merge pull request #204 from trptcolin/visitor_dispatch_caching

Cache visitor dispatch on a per-visitor basis
Support `columns_for_distinct`
fix one deprecation warning
Merge pull request #211 from yahonda/support_columns_for_distinct_oracle

Support `columns_for_distinct` with Oracle adapter
Release 4.0.1
Fix warning of shadowing variable
Add :encode_with for proper YAML serialization
Merge pull request #196 from vipulnsward/remove_deprecated

Remove deprecated
Remove deprecated calls to `delete` with preference to using `compile_delete` and then calling `to_sql` on the resulting object to execute the SQL
Remove deprecated calls to `insert` with preference to using `compile_insert` and then calling `to_sql` on the resulting object to execute the SQL
Remove deprecated calls to `update` with preference to using `compile_update` and then calling `to_sql` on the resulting object to execute the SQL
Remove deprecated calls to using `UpdateManager` without setting `UpdateManager#key`
Remove deprecated calls to using `where_clauses`