gemspec: Add minimum Ruby version
gemspec: Add minimum Ruby version

I think 2.2 might also work, but I was unable to get the dependencies
installed and didn't want to spend too much time on it. Ruby 2.2, 2.3,
and 2.4 have all been EOL'd. Versions earlier than 2.2 won't work
because `unicode_normalize` is a Ruby 2.2 feature.

academia-edu/dejunk,academia-edu/dejunk