Fix DRbUnknown error with unserializable Exception causes

[jdelStrother]

I ran into this when an error was raised in a view template, so got wrapped in ActionView::TemplateError. ActionView::TemplateError was correctly serialised, but its cause still referenced classes that don't exist in the sink, resulting in errors like this:

flatware-rspec-2.2.0/lib/flatware/rspec/formatters/console.rb:29:in `summarize': undefined method `pending_examples' for
#<DRb::DRbUnknown:0x0000000105f7c6b8 @name="User", @buf="\x04\bo: 
Flatware::RSpec::Checkpoint\x06:\f@events{\t:\x10deprecation[\x00:\x11
dump_pendingo:7Flatware::RSpec::Marshalable::ExamplesNotification\x06:\x0E
@reporterS:AFlatware::RSpec::Marshalable::ExamplesNotification::Reporter\b:\rexamples[\x

Maybe something like this to fix it?

[briandunn]

Nice!