Strip whitespace from depends

This was flagging our cookbooks incorrectly for FC007 due to the way we
specify long list of dependencies.

```ruby
%w[
  apt
  build-essential
  git
  python
  ark
  runit
  firewall
].each do |dep|
  depends dep
end
```

Having the names indented means they're actually strings like "  apt"
which is how the ast sees them.

```
  [0] <tstring_content value="  apt">
  <pos line="9" column="0"/>
```

The leading space doesn't actually matter though since knife strips the
whitespace when it's building metadata.json.

The correct place to fix this is probably in the AST but I think that's
way outside my skillset.