updated rails from 4.1.13 to 4.2.5. This is required for rails to make use of new features in postgres 9.5

@Ralf
TODO: migration /Users/groucho/Documents/HPI/workspace/Xikolo-CodeOcean/codeocean/db/migrate/20160204111716_add_user_to_code_harbor_link.rb
is broken. A reference is made to table users which does not exist.
Workaround: run rake db:schema:load before running db:migrate. Still this should be fixed.

db/migrate/20161214144837_create_lti_parameters.rb

@@ -4,9 +4,9 @@ class CreateLtiParameters < ActiveRecord::Migration
       t.string :external_user_id
       t.belongs_to :consumers
       t.belongs_to :exercises
-      t.column :lti_parameters, :jsonb
+      t.jsonb :lti_parameters, null: false, default: '{}'
 
       t.timestamps
     end
   end
-end
+end