UTF8 strings lowercase conversion

Currently, `process_text` converts the given string to lowercase in order to perform the word matching, but unfortunately Ruby does not convert UTF8 strings properly.
I've noticed inconsistencies like this one:

```ruby
require 'whatlanguage'
puts "ÂNCORA COR ÂMBAR".language # => spanish
puts "âncora cor âmbar".language # => portuguese
```

Thanks for the library!