Also warn on timestamp in datetime migration cop

The types `timestamp` and `datetime` are aliases:
https://github.com/rails/rails/blob/v4.2.10/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb#L362-L364

rubocop/cop/migration/datetime.rb

@@ -7,14 +7,18 @@ module RuboCop
       class Datetime < RuboCop::Cop::Cop
         include MigrationHelpers
 
-        MSG = 'Do not use the `datetime` data type, use `datetime_with_timezone` instead'.freeze
+        MSG = 'Do not use the `%s` data type, use `datetime_with_timezone` instead'.freeze
 
         # Check methods in table creation.
         def on_def(node)
           return unless in_migration?(node)
 
           node.each_descendant(:send) do |send_node|
-            add_offense(send_node, :selector) if method_name(send_node) == :datetime
+            method_name = node.children[1]
+
+            if method_name == :datetime || method_name == :timestamp
+              add_offense(send_node, :selector, format(MSG, method_name))
+            end
           end
         end
 
@@ -23,12 +27,14 @@ module RuboCop
           return unless in_migration?(node)
 
           node.each_descendant do |descendant|
-            add_offense(node, :expression) if descendant.type == :sym && descendant.children.last == :datetime
+            next unless descendant.type == :sym
+
+            last_argument = descendant.children.last
+
+            if last_argument == :datetime || last_argument == :timestamp
+              add_offense(node, :expression, format(MSG, last_argument))
             end
           end
-
-        def method_name(node)
-          node.children[1]
         end
       end
     end

spec/rubocop/cop/migration/datetime_spec.rb

@@ -9,6 +9,7 @@ describe RuboCop::Cop::Migration::Datetime do
   include CopHelper
 
   subject(:cop) { described_class.new }
+
   let(:migration_with_datetime) do
     %q(
       class Users < ActiveRecord::Migration
@@ -22,6 +23,19 @@ describe RuboCop::Cop::Migration::Datetime do
     )
   end
 
+  let(:migration_with_timestamp) do
+    %q(
+      class Users < ActiveRecord::Migration
+        DOWNTIME = false
+
+        def change
+          add_column(:users, :username, :text)
+          add_column(:users, :last_sign_in, :timestamp)
+        end
+      end
+    )
+  end
+
   let(:migration_without_datetime) do
     %q(
       class Users < ActiveRecord::Migration
@@ -58,6 +72,17 @@ describe RuboCop::Cop::Migration::Datetime do
       aggregate_failures do
         expect(cop.offenses.size).to eq(1)
         expect(cop.offenses.map(&:line)).to eq([7])
+        expect(cop.offenses.first.message).to include('datetime')
+      end
+    end
+
+    it 'registers an offense when the ":timestamp" data type is used' do
+      inspect_source(cop, migration_with_timestamp)
+
+      aggregate_failures do
+        expect(cop.offenses.size).to eq(1)
+        expect(cop.offenses.map(&:line)).to eq([7])
+        expect(cop.offenses.first.message).to include('timestamp')
       end
     end
 
@@ -81,6 +106,7 @@ describe RuboCop::Cop::Migration::Datetime do
   context 'outside of migration' do
     it 'registers no offense' do
       inspect_source(cop, migration_with_datetime)
+      inspect_source(cop, migration_with_timestamp)
       inspect_source(cop, migration_without_datetime)
       inspect_source(cop, migration_with_datetime_with_timezone)