Ignore directories when wildcards are used.

This simple change allows using wildcards without facing the error shown below:

```
fixtures2js: {
  tests: {
    files: {
      "test/fixtures.js": "test/fixtures/**/*",
    }
  }
}
```

```
Error: EISDIR, illegal operation on a directory
    at Object.fs.readSync (fs.js:476:19)
    at Object.fs.readSync (node_modules/karma/node_modules/graceful-fs/polyfills.js:218:23)
    at Object.fs.readFileSync (fs.js:310:28)
    at node_modules/grunt-fixtures2js/tasks/fixtures2js.js:17:50
```

Thanks for fixtures2js, it saved my day :-)