UI simplified for dependencies without version restriction

see: https://github.com/hmemcpy/Paket.VisualStudio/pull/7

I think that '>= 0' is useless info that makes UI more complicated.
Default `no restrictions` case is obvious.

![image](https://cloud.githubusercontent.com/assets/1197905/5395743/32a63578-815d-11e4-99fc-0635536896a5.png)