Use datetime type in migration

Author: ankane

db/migrate/20200205223315_install_blazer.rb

@@ -14,7 +14,7 @@ def change
       t.references :query
       t.text :statement
       t.string :data_source
-      t.timestamp :created_at
+      t.datetime :created_at
     end
 
     create_table :blazer_dashboards do |t|
@@ -39,7 +39,7 @@ def change
       t.text :slack_channels
       t.string :check_type
       t.text :message
-      t.timestamp :last_run_at
+      t.datetime :last_run_at
       t.timestamps null: false
     end
   end