Update version on travis.yml

.travis.yml

@@ -1,6 +1,6 @@
 language: ruby
 rvm:
-  - 2.3.3
+  - 2.4.1
 before_script:
   - cp config/database.travis.yml config/database.yml
   - mkdir -p public/app

db/migrate/20170518182029_add_polymorphic_constraints.rb

@@ -0,0 +1,7 @@
+class AddPolymorphicConstraints < ActiveRecord::Migration[5.1]
+  def change
+    add_polymorphic_constraints :pointer, :tool_slots
+    add_polymorphic_constraints :pointer, :plants
+    add_polymorphic_constraints :pointer, :generic_pointers
+  end
+end