Fix c4244 (facebook#49)

* (*pedantry alert*)
Right-shifting a 32-bit value by 32 is, strictly, undefined behavior,
although every compiler worth its salt will understand that this is
just a funny way to generate 0. But anyways, this code causes VC++ to
emit warning C4293, which, per Office policy, may not be disabled.

* Fix C4244 warnings in MethodCall.cpp when compiling for x86, caused
by implicitly truncating `int64_t` values to `int` values.

ReactCommon/cxxreact/MethodCall.cpp

@@ -64,8 +64,8 @@ std::vector<MethodCall> parseMethodCalls(folly::dynamic&& jsonData) {
     }
 
     methodCalls.emplace_back(
-      moduleIds[i].asInt(),
-      methodIds[i].asInt(),
+      static_cast<int>(moduleIds[i].asInt()),
+      static_cast<int>(methodIds[i].asInt()),
       std::move(params[i]),
       callId);