Don't compile dependencies with Traceur

Without this fix, Traceur will interpret dependencies as ES6 modules. Since those dependencies don't actually contain any ES6 export statements, they will then manifest as empty modules.

With this fix, you can then do things like `var _ = require("underscore")` inside your ES6 tests or system-under-test, and it will actually work. I can upload an example project if this doesn't make sense.

(An alternate approach would be to add a second file, e.g. `no-dependencies`, which does this, so that people could do

```
mocha --compilers js:mocha-traceur/no-dependencies test/*.js
```

to avoid compiling dependencies. But I think not-compiling them by default is better, at least in the current climate.)