Use nanoseconds precision subtracting Time objects

To be consistent with MRI, let's return a Float with nanoseconds
precision when we are subtracting two Time objects.

We simply need to rely on the instance's nsec method instead of the usec
one. Thus, we multiply the subtraction of each object's nano-seconds
with 1e-9 instead of 1e-6.
Added specs for nsec precision subtracting Time objects
Merge pull request #3071 from robin850/minus-nsec-precision

Nanosecond precision with Time#-