Ignores non .js files

Useful when testing modules that includes .hbs files or text files. Coverify is trying to instrument those files today, failing with an error.

Example of the use case
```javascript
// module.js file
var template = require('./template.hbs');
console.log(template({name: 'Fernando'}));

// template.hbs file
Hi {{name}}!
```

Execute coverify
```bash
browserify -t coverify ./module.js
```