Fixed problem with unused parameters.

I pulled down this project just a couple days ago, ran into this error compiling against nightly:

src/ast.rs:215:9: 215:52 error: this function takes 3 parameters but 6 parameters were supplied
src/ast.rs:215         visit::walk_struct_def(self, s, i, g, n, e)
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/ast.rs:215:9: 215:31 error: cannot determine a type for this bounded type parameter: unconstrained type
src/ast.rs:215         visit::walk_struct_def(self, s, i, g, n, e)
                       ^~~~~~~~~~~~~~~~~~~~~~
make: *** [all] Error 101

Apparently the i,g,n parameters are optional/ignored now. This quick fix got it compiling again.

Thanks for the great work so far with racer!