require('./package.json') causes coverify transform to fail

Doing something like:

```js
var pkg = require('./package.json')

module.exports = function (cb) {
    var i = 0;
    var iv = setInterval(function () {
        if (i++ === 10 || (false && neverFires())) {
            clearInterval(iv);
            cb(null, 111);
        }
    }, 10);
};
```

in `foo.js` and then running:

```
$ node_modules/browserify/bin/cmd.js -t coverify test.js --bare
```

Will output:
```
Error: Unexpected token :

/Users/drewp/dev/coverify-test/package.json:2
  "name": "coverify-test",
        ^
```

This is because the coverify transform is attempting to parse the `package.json` as JS and not JSON. But it's weird that a transform would have to worry about every single type of file passing through the pipeline. Should this check happen within coverify, or within browserify itself?