Fix field_error_proc for Rails 4 TagHelper changes

`ActionView::Base::InstanceTag` is replaced by a new hierarchy of Tag
classes under `ActionView::Helpers::Tags`.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Ensure Project#to_param always returns a String

Signed-off-by: Alex Coles <alex@alexbcoles.com>