Remove Xcode 6.4, add Xcode 10

Author: martinmoene

.travis.yml

@@ -133,24 +133,24 @@ matrix:
           sources: *apt_sources
 
     - os: osx
-      env: COMPILER=clang++
-      compiler: clang++
-      osx_image: xcode6
-
-    - os: osx
-      env: COMPILER=clang++
-      compiler: clang++
-      osx_image: xcode7
+      osx_image: xcode7.3
+      compiler: clang
+      env: COMPILER='clang++'
 
     - os: osx
-      env: COMPILER=clang++
-      compiler: clang++
       osx_image: xcode8
+      compiler: clang
+      env: COMPILER='clang++'
 
     - os: osx
-      env: COMPILER=clang++
-      compiler: clang++
       osx_image: xcode9
+      compiler: clang
+      env: COMPILER='clang++'
+
+    - os: osx
+      osx_image: xcode10
+      compiler: clang
+      env: COMPILER='clang++'
 
 script:
   - export CXX=${COMPILER}