Update instrumenter.js

I had problem using your lib, and it seems I found a solution. 
In traceur: https://github.com/google/traceur-compiler/blob/master/src/Compiler.js#L132 you can pass the filename argument to compile method.
I also remove ```this._compiler.options_.filename = filename;``` which seems to do nothing (also based on https://github.com/google/traceur-compiler/blob/master/src/Compiler.js)