Bugfix: NoMethodError: undefined method `<' for nil:NilClass

When I run

```ruby
GeoPattern.generate('How do I get started', :color => '#7A3332')
```

I get the following error

```
NoMethodError: undefined method `<' for nil:NilClass
from /opt/boxen/rbenv/versions/2.2.0/lib/ruby/gems/2.2.0/gems/geo_pattern-1.3.0/lib/geo_pattern/pattern_generator.rb:87:in `generate_pattern'
```

This PR fixes the issue for me, let me know if you want any changes.

Thanks!