ruby: change 'DBMS::Base#run_sql()' to be more generic
ruby: remove 'history_table_empty?()' method
ruby: refactor 'DBMS::PostgreSQL#execute_sql()'
ruby: define 'DBMS::Base#_history_table_statement' and change '#create_history_table()' to call it
ruby: change 'DBMS::Base#get_migrations()' to call '#_get_migrations()' with default args