fixed: include boost header as necessary

Without this include, sometimes BOOST_VERSION was defined and sometimes
it was not, depending on which includes came before it. The result was a
random mix of sleep or sleep_for for boost versions >= 1.50.

src/util.h

@@ -20,6 +20,7 @@
 #include <vector>
 #include <string>
 
+#include <boost/version.hpp>
 #include <boost/thread.hpp>
 #include <boost/filesystem.hpp>
 #include <boost/filesystem/path.hpp>