metadata source should not write a constraint in Berksfile.lock

There is a related Issue to #623. The locked version is no longer written, but the constraint still is. This results in the same behavior as before:

```
$ bundle exec berks install
The cookbook downloaded from path: '.':
  nexus (11.0.0)

does not satisfy the version constraint:
  nexus (= 10.0.0)

This occurs when the Chef Server has a cookbook with a missing/mis-matched version number in its `metadata.rb`
```

We also need to be careful about reading a Berksfile generated by an older version of Berkshelf who has an entry for the metadata containing a constraint and/or a locked_version. The end user would need to destroy their Berksfile.lock to stop receiving this error message.

This fix should be based off of 2-0-stable and then imported into master