We were adding each year in the translation header and that was
generating long translator comments with a lot of years. Instead of add
every year in the header, this patch changes that and use the format:

translator <email>, FIRST_YEAR-CURRENT_YEAR.

This method lost some information, because now we have a range, and
before we've a complete list that can have gaps, like "2015, 2017,
2018.". Now that example will be "2015-2018".

Fix #8