Nanosecond precision with Time#-

Hello,

This pull request aims at making `Time#-` consistent with MRI when we are passing another `Time` object. Currently, Rubinius returns the µsec precision while MRI returns a nsec one so we just need to rely on the object's `nsec` method instead of `usec` ; thus, we need to multiply the difference with 1e-9 instead of 1e-6.

For the record, this problem has been discovered because Rubinius fails [here](https://github.com/rails/rails/blob/9c492885d178b1a3d0de2f710a8a276db3d867b2/activesupport/test/core_ext/time_with_zone_test.rb#L252) in the Rails' test suite.

Have a nice day.