fix compilation error on Apple OS X Mavericks

The was an issue in the code that prevented successful
compilation on OS X Mavericks.

Link: https://gist.github.com/sjackman/8106970
Reported-by: <sjackman@gmail.com>
Reported-by: Bede Constantinides <bede.constantinides@manchester.ac.uk>
Reported-by: François Olivier Gagnon-Hébert <francois-olivier.gagnon-hebert.1@ulaval.ca>
Link: https://github.com/sebhtml/ray/issues/225
Reviewed-by: <sjackman@gmail.com>
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>