fix slug processing to allow for numeric tags

It appears that the YAML parsing is smart enough to see that something that is entirely numeric should be a number. For example:

    Tags:
    - 2013
    - 2014

However, the slugify method assumed that the `text` being passed to it was actually an object with the `lower` method. In some cases, such as those above, it could be an `int` which causes `engineer build` to fail. By casting `text` to `unicode` we avoid this problem.