Fix get version gemfile regexp and add tests

This hopefully fixes #359 by matching both end of line characters `\r\n` and `\n`.  Also added a fix for  matching variable length version numbers like `1.2` instead of just ones of length three like `1.2.3`.  

Do we need to support formats like these? 
``` 
haml (>= 3.0, < 5.0)
mime-types (>= 1.16)
ruby_parser (~> 3.1)
```