Remove privatizing of Fixnum#/ from assert_distance_of_time_in_words

MRI reimplemented Date in C so it doesn't hit this division anymore
while JRuby still uses the old stdlib implementation of Date so
it will always hit this.

With this change the actionview date_helper_test.rb tests should pass on JRuby.

The privatizing of Fixnum#/ was introduced in https://github.com/rails/rails/commit/dc3ac357dc21c8ba05de95760da2b2c9f963012e.

#11700

cc: @arunagw, @rafaelfranca