I'm trying to upgrade to Rails 4.2 from Rails 4.1.9 and when I attempt to run the console or the server i get the following error. Any thoughts? I thought the html-scanner gem was included with Action::View
gems/activesupport-4.2.0/lib/active_support/dependencies.rb:274:in `require': cannot load such file -- action_view/vendor/html-scanner (LoadError)
The OP's issue (and mine) was to do with an incompatibility in the prototype-rails gem. This can be fixed by using the "4.2" branch of prototype-rails directly from github. Modify your Gemfile entry as follows:
gem 'prototype-rails', github: 'rails/prototype-rails', branch: '4.2'
Note that the prototype-rails gem will not be supported from Rails 5 onwards, so now would be as good a time as any to remove this dependency.