Re-enable sanitizers in tests for XCode 7.3 on OS X.
They will most likely fail, this is just to see the effect.
diff --git a/.travis.yml b/.travis.yml
index 8573933..c78ef0d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -128,8 +128,7 @@
       compiler: clang
       # OS X 10.11
       osx_image: xcode7.3
-      # ASAN is disabled because AppleClang does not support -fsanitize=undefined.
-      env: COMPILER=clang-default STL=libc++ NO_ASAN=1
+      env: COMPILER=clang-default STL=libc++
 
 # Disabled combinations:
 #