Travis: Distribute OS X builds more evenly (for wait/queuing purposes).

.travis.yml

@@ -106,6 +106,11 @@ matrix:
           packages: g++-4.7
       compiler: gcc-4.7
 
+    - os: osx
+      env: COMPILER=g++-6 CCOMPILER=gcc-6
+      osx_image: xcode7.3
+      compiler: g++-6
+
     - os: linux
       env: DIST=trusty COMPILER=g++-4.8 CCOMPILER=gcc-4.8
       sudo: required
@@ -126,11 +131,6 @@ matrix:
           packages: g++-4.9
       compiler: gcc-4.9
 
-    - os: osx
-      env: COMPILER=g++-6 CCOMPILER=gcc-6
-      osx_image: xcode7.3
-      compiler: g++-6
-
     - os: linux
       env: DIST=trusty COMPILER=g++-5 CCOMPILER=gcc-5
       sudo: required
@@ -141,6 +141,11 @@ matrix:
           packages: g++-5
       compiler: gcc-5
 
+    - os: osx
+      env: COMPILER=clang++ CCOMPILER=clang
+      osx_image: xcode7.3
+      compiler: clang
+
     - os: linux
       env: DIST=trusty COMPILER=g++-6 CCOMPILER=gcc-6
       sudo: required
@@ -151,11 +156,6 @@ matrix:
           packages: g++-6
       compiler: gcc-6
 
-    - os: osx
-      env: COMPILER=clang++ CCOMPILER=clang
-      osx_image: xcode7.3
-      compiler: clang
-
     - os: linux
       env: CXXLIB=libstdc++ COMPILER=clang++-3.7 CCOMPILER=clang-3.7
       addons: