Improve test stubbing

Signed-off-by: David Celis <me@davidcel.is>
Add option to whitelist certain gem versions

Add an option, `version_whitelisted_license` which allows the user to
specify _one_ license which, when used, will ignore gem versions. This
is useful for gems written in house that will always have licenses
acceptable to the company. **It should not be used for public gems**.

```ruby
config.version_whitelisted_license = 'New Relic'
```

Then:

```yaml
new_relic_gem: # Omit the gem version.
  license: New Relic
  project_url: # ...
  license_url: # ...
```

Now, as you bundle update that gem, you won't have to modify the papers
manifest.

Signed-off-by: David Celis <me@davidcel.is>