checking dictionaries after speller is created

When a aspell dictionary is not installed, this library normaly does a core dump. 
These changes are comparing the available dictionaries with the selected one. 
It now raises an error when a dictionary is not availbale.
What's the rationale for using `Array#any?` here? Using just `available_dictionaries.include?(dictionary)` is simpler, probably faster and should produce the same outcome anyway.