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.
Merge pull request #11771 from atambo/jruby_assert_distance_of_time_in_words

Remove privatizing of Fixnum#/ from assert_distance_of_time_in_words