fix updating variable in #insertConfig()

When trying to update a config block that was a variable reference, an error was being thrown:

```
/Users/eddie/Dropbox/Sites/OSS/yeoman/generator/node_modules/gruntfile-editor/index.js:33
    .key(name).value(config);
               ^
TypeError: Object #<Object> has no method 'value'
    at GruntfileEditor.insertConfig (/Users/eddie/Dropbox/Sites/OSS/yeoman/generator/node_modules/gruntfile-editor/index.js:33:16)
    at module.exports (/Users/eddie/Dropbox/Sites/OSS/yeoman/generator-angular/helpers/gruntfile.js:16:18)
    at module.exports.yeoman.generators.Base.extend.writing.build (/Users/eddie/Dropbox/Sites/OSS/yeoman/generator-angular/generators/app/index.js:255:17)
    at /Users/eddie/Dropbox/Sites/OSS/yeoman/generator/lib/base.js:387:14
    at processImmediate [as _immediateCallback] (timers.js:336:15)
```