Travis: Fix C++ compilation

Summary:
@public

Removes the usage of C++14 features, and enforces C++11 via compiler flags.

Reviewed By: SidharthGuglani

Differential Revision: D15334938

fbshipit-source-id: 011764b5f226fef6a35e0c7c1dd170a39ae2261e

testutil/testutil.cpp

@@ -9,8 +9,6 @@
 #include <yoga/YGNode.h>
 #include <yoga/event/event.h>
 
-#include <utility>
-
 namespace facebook {
 namespace yoga {
 namespace test {
@@ -48,7 +46,9 @@ int TestUtil::nodeCount() {
 
 int TestUtil::stopCountingNodes() {
   Event::reset();
-  return std::exchange(nodeInstanceCount, 0);
+  auto prev = nodeInstanceCount;
+  nodeInstanceCount = 0;
+  return prev;
 }
 
 ScopedEventSubscription::ScopedEventSubscription(

tools/build_defs/oss/yoga_defs.bzl

@@ -58,6 +58,7 @@ BASE_COMPILER_FLAGS = [
     "-Wall",
     "-Werror",
     "-O2",
+    "-std=c++11",
 ]
 
 LIBRARY_COMPILER_FLAGS = BASE_COMPILER_FLAGS + [