Fix DateTime.civil_from_format to work with Ruby 2.0

TimeExtCalculationsTest had 2 failing tests on ruby-2.0.0-p247:
test_local_time and test_time_with_datetime_fallback

For some reason Fixnum#to_r method is returning Fixnum instead of
Rational which results in zero offset after division.

This patch works around the problem by using explicit call to
Rational constructor.