Support 64-bit precision all around

32 bits of precision is not enough; it's entirely possible to run out
in a mere mortal lifetime.

Testing methodology: set the initial sequence to an integer beyond 32
bits of precision, so that every test will stress code paths involving
such larger integers, e.g. to catch anything using Fixnums on a 32-bit
build of Ruby.

It seems unlikely that a defect would arise only in cases where
numbers require limited precision and such bugs would appear right
away after starting to use queue_classic, so it seems that the loss of
testing those case is acceptable.

Signed-off-by: Daniel Farina <daniel@heroku.com>
Merge pull request #120 from fdr/64-bit

Support 64-bit precision all around