Add UTF-8 encoding at head of files to get build green (incompatible encoding regexp match)

bundle exec rake failed before my pull request like this:

    1) WordsCounted::Counter words does not split on unicode chars
      Failure/Error: counter = Counter.new("S?o Paulo")
      Encoding::CompatibilityError:
        incompatible encoding regexp match (ASCII-8BIT regexp with UTF-8 string)
      # ./lib/words_counted/counter.rb:16:in `scan'
      # ./lib/words_counted/counter.rb:16:in `initialize'
      # ./spec/words_counted/counter_spec.rb:55:in `new'
      # ./spec/words_counted/counter_spec.rb:55:in `block (3 levels) in <module:WordsCounted>'