Replace deprecated AR table_exists? API in Rails 5.

A propos, it's about time we simplify context.rb into separate files
for Active Record 4.x and 5.0 like we did for 3.x, which will enable
removing a fair number of conditionals.

lib/ransack/adapters/active_record/context.rb

@@ -25,7 +25,7 @@ def type_for(attr)
           name         = attr.arel_attribute.name.to_s
           table        = attr.arel_attribute.relation.table_name
           schema_cache = @engine.connection.schema_cache
-          unless schema_cache.table_exists?(table)
+          unless schema_cache.send(database_table_exists?, table)
             raise "No table named #{table} exists."
           end
           schema_cache.columns_hash(table)[name].type
@@ -134,6 +134,14 @@ def alias_tracker
 
         private
 
+        def database_table_exists?
+          if ::ActiveRecord::VERSION::MAJOR >= 5
+            :data_source_exists?
+          else
+            :table_exists?
+          end
+        end
+
         def get_parent_and_attribute_name(str, parent = @base)
           attr_name = nil