Fix warnings when compiling with VS2013 Nov CTP

These two lines caused a C4800 warning when compiling with the Nov 2013 Visual C++ compiler CTP.
(e.g. selene\primitives.h(58): warning C4800: 'int' : forcing value to bool 'true' or 'false' (performance warning))

Thanks for Selene by the way, by far the most readable and convenient approach to C++/Lua integration I've seen to date.