Fix libc++ use

Summary: Use of std::max from libc++ resulted in error.
std::max found type of its arguments diffrent
whereas it expected them to be same.
This diff fix the error.
Closes #223

Reviewed By: @yfeldblum

Differential Revision: D2144639

Pulled By: @sgolemon

folly/io/async/AsyncServerSocket.cpp

@@ -726,8 +726,8 @@ void AsyncServerSocket::handlerReady(
 
     std::chrono::time_point<std::chrono::steady_clock> nowMs =
       std::chrono::steady_clock::now();
-    int64_t timeSinceLastAccept = std::max(
-      int64_t(0),
+    auto timeSinceLastAccept = std::max<int64_t>(
+      0,
       nowMs.time_since_epoch().count() -
       lastAccepTimestamp_.time_since_epoch().count());
     lastAccepTimestamp_ = nowMs;