Bug fix: limit _read() size to 2GB instead of 4GB.

The maximum number of bytes read was incorrectly set to 4GB (2^32)
instead of 2GB (2^31).

The result was that language models over 2GB failed to load in memory on
Windows.
Merge pull request #16 from mbardea/master

Bug fix: limit _read() size to 2GB instead of 4GB.