Workaround for failed require of 'coverage-reporter.js' for node tests

Testing with node 0.10.7:

```
> language-tags@0.0.1 test /Users/matt/Dev/language-tags
> node_modules/buster/bin/buster-test -c tests/buster.js

Failed requiring ./coverage/coverage-reporter.js: Cannot find module '/Users/matt/Dev/language-tags/coverage/coverage-reporter.js'
```

The workaround is to only pass `options.path` to `addFileResource` when running in the browser environment.