Fix for Holidays::UnknownRegionError for German regions without exceptional holidays

Fix for Holidays::UnknownRegionError 
```ruby
2.1.1 (main):0 > Holidays.on(Date.civil(2009,10,3), :de_be)
Holidays::UnknownRegionError: Could not load holidays/de_be
```

```ruby
2.1.1 (main):0 > Holidays.on(Date.civil(2009,10,3), :de_by)
=> [{:date=>Sat, 03 Oct 2009,
  :name=>"Tag der Deutschen Einheit",
  :regions=>[:de]}]
```

DE-BE, DE-HB, DE-HH, DE-NI, DE-SH are defined by ISO_3166-2:DE http://en.wikipedia.org/wiki/ISO_3166-2:DE.

These states do not have any exceptional holidays - thus they are the common denominator for public holidays in Germany. 

Since I store the state (e.g. Berlin) to my user for which I want to query the available holidays I added all states to defined_regions to circumvent the UnknownRegionError .