Fixed structure validator (recognizes puma as allowed web server).

The problem was in the ```valid?``` method, which does not recognize ```puma``` as allowed web server so ```shelly add``` and ```shelly check``` received ```false``` from ```valid?``` method

![gemfile](https://f.cloud.github.com/assets/619324/313291/b7f3a54a-97a7-11e2-835c-321a4cb71e88.PNG)
![add](https://f.cloud.github.com/assets/619324/313293/ba40bd7e-97a7-11e2-9fe1-e339030da4df.PNG)
![check](https://f.cloud.github.com/assets/619324/313294/bc360044-97a7-11e2-8ea1-a2c699282886.PNG)