backtrace not cleaned in minitest bundled with ruby 2.1.2

Hello,

In the minitest that is bundled with ruby 2.1.2, the backtrace cleaner doesn't seem to be doing its job because I get lines like the following in assertion failure backtraces. :cold_sweat: 

Thanks for your consideration. 

```
    /usr/lib/ruby/2.1.0/minitest/unit.rb:1265:in `run'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:940:in `block in _run_suite'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:933:in `map'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:933:in `_run_suite'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:914:in `block in _run_suites'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:914:in `map'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:914:in `_run_suites'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:884:in `_run_anything'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:1092:in `run_tests'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:1079:in `block in _run'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:1078:in `each'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:1078:in `_run'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:1066:in `run'
    /usr/lib/ruby/2.1.0/minitest/unit.rb:802:in `block in autorun'
```