Don't halt callbacks on return false

This will be the default behaviour in Rails 5.1, and it's a much better
approach.

I've checked the code and luckily there doesn't seem to be a single
place where we could accidentally stop the callback chain by returning
false in (for example) a `before_save` callback.

config/initializers/callback_terminator.rb

@@ -0,0 +1 @@
+ActiveSupport.halt_callback_chains_on_return_false = false