Use Ridders' method in GradientChecker.

Using Ridders' method gives orders of magnitude more accuracy compared
to central differences. This will make things slower, but this is
primarily a testing/debugging feature and the speed hit is not a
concern. This should also reduce the false positive rates when users
enable check_gradients. This is reflected the increased sensitivity of
the tests for GradientChecker.

https://github.com/ceres-solver/ceres-solver/issues/554

Change-Id: I6b871c72df55be1c31175ba062cf3c1e94e4b662