Check if requirement is set before matching version.

If requirement is nil and you try to .match a regex it will error:

```
[Librarian]                                                                 Checking sensu/0.5.0 <https://github.com/sensu/sensu-puppet.git#master>
/Library/Ruby/Gems/1.8/gems/librarian-puppet-0.9.8/lib/librarian/puppet/requirement.rb:23:in `range_requirement?': undefined method `match' for nil:NilClass (NoMethodError)
	from /Library/Ruby/Gems/1.8/gems/librarian-puppet-0.9.8/lib/librarian/puppet/requirement.rb:11:in `gem_requirement'
```

sensu-puppet specifies a dependency without a version `dependency 'puppetlabs/apt'`