When no regions are loaded, accessing a subregion fails.

This problem was already addressed by pull request #64 by [Goltergaul](https://github.com/Goltergaul). However, his tests were failing for Ruby 1.8.7, and since I needed this for a project that runs under 1.8.7 I had to fix them anyway. Also, I thought I'd try for code that is a little closer to your style of writing - whether I have achieved this remains for you to decide.

In order to prove beforehand that the tests pass, I have submitted my fork to Travis. You can view the build results for the branch this pull request comes from [over there](https://travis-ci.org/capita/holidays/builds/35971115).

Please note that the problem addressed in pull request #71 by [rojoko](https://github.com/rojoko) also gets solved by this fix. 

To restate the problems:

* The holiday definition for a region is not loaded implicitly when you try to access one of its subregions that has a holiday definition of its own:

```
Christians-MacBook-Pro-2:holidays csage$ git co 9b69c9b
...
HEAD is now at 9b69c9b... SG: add stray April Fool's informal
Christians-MacBook-Pro-2:holidays csage$ bundle exec irb
2.1.3 :001 > require 'holidays'
 => true
2.1.3 :002 > Date.today.holiday? :de_bb
Holidays::UnknownRegionError: Could not load holidays/de_bb
	from /Users/csage/Projects/holidays/lib/holidays.rb:360:in `rescue in block in parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:357:in `block in parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:355:in `each'
	from /Users/csage/Projects/holidays/lib/holidays.rb:355:in `parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:320:in `parse_options'
	from /Users/csage/Projects/holidays/lib/holidays.rb:122:in `between'
	from /Users/csage/Projects/holidays/lib/holidays.rb:68:in `on'
	from /Users/csage/Projects/holidays/lib/holidays.rb:445:in `holidays'
	from /Users/csage/Projects/holidays/lib/holidays.rb:455:in `holiday?'
	from (irb):2
	from /Users/csage/.rvm/rubies/ruby-2.1.3/bin/irb:11:in `<main>'
2.1.3 :003 > Date.today.holiday? :de
 => false
2.1.3 :004 > Date.today.holiday? :de_bb
 => false
```

* Trying to access a subregion with no holiday specifications of its own fails. That is true even after the containing region has been loaded:

```
Christians-MacBook-Pro-2:holidays csage$ git co 9b69c9b
...
HEAD is now at 9b69c9b... SG: add stray April Fool's informal
Christians-MacBook-Pro-2:holidays csage$ bundle exec irb
2.1.3 :001 > require 'holidays'
 => true
2.1.3 :002 > Date.today.holiday? :de_be
Holidays::UnknownRegionError: Could not load holidays/de_be
	from /Users/csage/Projects/holidays/lib/holidays.rb:360:in `rescue in block in parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:357:in `block in parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:355:in `each'
	from /Users/csage/Projects/holidays/lib/holidays.rb:355:in `parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:320:in `parse_options'
	from /Users/csage/Projects/holidays/lib/holidays.rb:122:in `between'
	from /Users/csage/Projects/holidays/lib/holidays.rb:68:in `on'
	from /Users/csage/Projects/holidays/lib/holidays.rb:445:in `holidays'
	from /Users/csage/Projects/holidays/lib/holidays.rb:455:in `holiday?'
	from (irb):2
	from /Users/csage/.rvm/rubies/ruby-2.1.3/bin/irb:11:in `<main>'
2.1.3 :003 > Date.today.holiday? :de
 => false
2.1.3 :004 > Date.today.holiday? :de_be
Holidays::UnknownRegionError: Could not load holidays/de_be
	from /Users/csage/Projects/holidays/lib/holidays.rb:360:in `rescue in block in parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:357:in `block in parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:355:in `each'
	from /Users/csage/Projects/holidays/lib/holidays.rb:355:in `parse_regions'
	from /Users/csage/Projects/holidays/lib/holidays.rb:320:in `parse_options'
	from /Users/csage/Projects/holidays/lib/holidays.rb:122:in `between'
	from /Users/csage/Projects/holidays/lib/holidays.rb:68:in `on'
	from /Users/csage/Projects/holidays/lib/holidays.rb:445:in `holidays'
	from /Users/csage/Projects/holidays/lib/holidays.rb:455:in `holiday?'
	from (irb):4
	from /Users/csage/.rvm/rubies/ruby-2.1.3/bin/irb:11:in `<main>'
```

Having the possibility to directly access the holidays for a specific subregion irrespective of its having any would be very convenient in the context of federal states, where address data usually includes the state (= the subregion). 

Both problems go away with this pull request, the feature desired by rojoko and myself becomes a one-liner:

```
Christians-MacBook-Pro-2:holidays csage$ git co subregion_load
...
Switched to a new branch 'subregion_load'
Christians-MacBook-Pro-2:holidays csage$ bundle exec irb
2.1.3 :001 > require 'holidays'
 => true
2.1.3 :002 > Date.today.holiday? :de_be
 => false
```

My apologies for the mess I made in the comments of the pull requests I have referenced here. Before this I sadly lacked experience with creating pull requests, so it took me several tries to get it even approximately right.