Use int conversion for Option class

The current implementation generates warnings on MSVC. However, we have
no real use cases for double-typed UCI option values now. Also parameter
tuning only accepts following three types:

  int, Value, Score

closes https://github.com/official-stockfish/Stockfish/pull/4505

No functional change

src/uci.h

@@ -61,7 +61,7 @@ public:
 
   Option& operator=(const std::string&);
   void operator<<(const Option&);
-  operator double() const;
+  operator int() const;
   operator std::string() const;
   bool operator==(const char*) const;