Add include for std::free from <cstdlib>

I was getting an error about `free` being undefined, so included and used the function from `<cstdlib>`. It seems to be the same as the C function.
```
../deps/sol/sol/demangle.hpp:44:5: error: use of undeclared identifier 'free'
    free(unmangled);
    ^
```
My compiler version: `Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)`