Ditch the dependency on the optional submodule, nobody's ever gonna care about it...

.gitmodules

@@ -1,6 +1,3 @@
 [submodule "Catch"]
 	path = Catch
 	url = https://github.com/philsquared/Catch.git
-[submodule "Optional"]
-	path = Optional
-	url = https://github.com/ThePhD/Optional.git

sol/object.hpp

@@ -22,11 +22,11 @@
 #ifndef SOL_OBJECT_HPP
 #define SOL_OBJECT_HPP
 
-#include "optional.hpp"
 #include "reference.hpp"
 #include "stack.hpp"
 #include "userdata.hpp"
 #include "variadic_args.hpp"
+#include "optional.hpp"
 
 namespace sol {
 

sol/optional.hpp

@@ -22,11 +22,12 @@
 #ifndef SOL_OPTIONAL_HPP
 #define SOL_OPTIONAL_HPP
 
+#include "compatibility.hpp"
 #include "in_place.hpp"
 #if defined(SOL_USE_BOOST)
 #include <boost/optional.hpp>
 #else
-#include "../Optional/optional.hpp"
+#include "optional_implementation.hpp"
 #endif // Boost vs. Better optional
 
 namespace sol {