re-enabling builds on travis for xcode 6.4 and 7.3
diff --git a/.travis.yml b/.travis.yml
index 25ed040..c9487eb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -183,6 +183,16 @@
           packages: ["clang-7", "valgrind", "libc6-dbg", "g++-6"]
           sources: *apt_sources
 
+    # Xcode 6.4 Clang
+    - env: COMPILER=clang++ CMAKE_OPTIONS="-DDOCTEST_THREAD_LOCAL=static"
+      osx_image: xcode6.4
+      os: osx
+
+    # Xcode 7.3 Clang
+    - env: COMPILER=clang++ HAS_ASAN=true CMAKE_OPTIONS="-DDOCTEST_THREAD_LOCAL=static"
+      osx_image: xcode7.3
+      os: osx
+
     # Xcode 8 Clang
     - env: COMPILER=clang++ HAS_TSAN=true # no HAS_ASAN - errors since using thread_local even in single-threaded cases - see errors: https://travis-ci.org/onqtam/doctest/builds/417181981
       osx_image: xcode8