Fix mutant specs

I ran into some weird issues running the underscore_spec file with Mutant. It would start running and then report things like
```
Spec file(s): "spec/unit/_i_n_f_l_e_c_t_o_/class_methods/underscore_spec.rb" not found for Inflecto.underscore:/Users/mossity/code/inflecto/lib/inflecto.rb:43:91a2d
Spec file(s): "spec/unit//i/n/f/l/e/c/t/o//class_methods/underscore_spec.rb" not found for Inflecto.underscore:/Users/mossity/code/inflecto/lib/inflecto.rb:43:5f8ef
```

Like the inflection was infecting the spec path or something? Not sure, but guessing you might have a clue what's going on here.