bump rails 5.1

Change-Id: Id0e0ad36c03cb52eb9ec158318a456386b169679
Reviewed-on: https://gerrit.instructure.com/122209
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

Gemfile.d/app.rb

@@ -18,7 +18,7 @@
 if CANVAS_RAILS5_0
   gem 'rails', '5.0.4'
 else
-  gem 'rails', '5.1.2'
+  gem 'rails', '5.1.3'
 end
 
 gem 'rack', '2.0.3'

app/models/tableless.rb

@@ -38,7 +38,7 @@ class Tableless < ActiveRecord::Base
     end
 
     def column(name, sql_type = nil, default = nil, null = true)
-      args = [name.to_s, default, connection.lookup_cast_type(sql_type.to_s),
+      args = [name.to_s, default, connection.send(:lookup_cast_type, sql_type.to_s),
               sql_type.to_s, null]
       columns << ActiveRecord::ConnectionAdapters::Column.new(*args)
     end