The current Rails::BacktraceCleaner only handles gems installed
in Gem.default_dir, not any of the alternative locations (~/.gem)
or in vendor/gems.
The attached patch handles those cases exactly as the
default_dir case, by generating a backtrace entry in the format
suggested here and
already used in the code. There is a minor additon for vendored
gems, which get a "[v]" after the version number.