Not compatible anymore with older ruby versions.

[alagos]

After these changes, I'm getting this error using ruby 2.4:

lib/ice_cube/schedule.rb:307: syntax error, unexpected ']'
        ences.last : occurrences[-n..]

lib/ice_cube/schedule.rb:448: syntax error, unexpected keyword_rescue, expecting keyword_end
                rescue StopIteration
                      ^

It would be good to update the gemspec and define what's the required ruby version for this gem.