Previous regex matcher let gems slip through if a custom path was
provided to bundler (e.g. `bundle install --path ../bundle`). Remove any
explicit path dependencies on potential ruby version based directory
names.

Generalize the gem backtrace exclusion.

…ssary

Fallback to object.method when necessary

Ruby 2.1 Module#prepend isn't private

It relies upon CallerFilter but wasn't requiring it.

[ci skip]