feature/fix-diff-version

Fixes invalid SemVer Version error that was caused by chef environment version constraints being of the form `= 0.1.2`. Version number is now pulled from chef environment version constraint and passed to cookbook object creation. 

```
Alexs-sMackBook-Pro-124:master_cookbook atrull$ head-chef env diff
Using Opscode organization 'mdsol-green'...
Loading environment distro from chef server...
Using Opscode organization 'mdsol-green'...
Loading cookbooks from berkshelf...
Calculating diff...
/Users/atrull/.rbenv/versions/1.9.3-p286/lib/ruby/gems/1.9.1/gems/semantic-1.2.1/lib/semantic/version.rb:8:in `initialize': = 0.1.14 is not a valid SemVer Version (http://semver.org) (ArgumentError)
```