Ruby: Correct set LONG_BITS and INDEX_SHIFT in class Set.

Ruby: Correct set LONG_BITS and INDEX_SHIFT in class Set.
The code for Set class was written under the assumption that
std::numeric_limits<long>::digits returns the number of bits used for
data type long, which was presumed to be either 32 or 64. But return value
is actually one less, that is, it is either 31 or 63. The value is now
being incremented by 1 so as to correctly set it.