Fixes compatibility with pry 0.10.x

Pry 0.10.x introduced Pry::Output class, due to class loading it conflicts with BetterErrors::REPL::Pry::Output, so let’s be more explicit about which class we use.

This fixes #255.
Fully qualified class name.
Merge pull request #258 from fredwu/pry_0.10.x_compatibility_fix

Fixes compatibility with pry 0.10.x