.travis.yml: Adding hacks for building with gcc on macOS.

.travis.yml

@@ -60,25 +60,24 @@ matrix:
       env:
         - MATRIX_EVAL="CC=clang-3.8 CXX=clang++-3.8"
       compiler: clang
-    # osx with xcode8.3/gcc
+    # osx with gcc6
     - os: osx
-      osx_image: xcode8.3
-      compiler: gcc
+      env:
+        - MATRIX_EVAL="brew install gcc6 && CC=gcc-6 && CXX=g++-7"
+    # osx with gcc7
+    - os: osx
+        - MATRIX_EVAL="brew install gcc7 && CC=gcc-7 && CXX=g++-7"
+    # osx with gcc8
+    - os: osx
+        - MATRIX_EVAL="brew install gcc8 && CC=gcc-8 && CXX=g++-8"
     # osx with xcode8.3/clang
     - os: osx
       osx_image: xcode8.3
       compiler: clang
-    # osx with xcode9.2/gcc
-    - os: osx
-      osx_image: xcode9.2
-      compiler: gcc
     # osx with xcode9.2/clang
     - os: osx
       osx_image: xcode9.2
       compiler: clang
-    # osx with xcode/gcc (default)
-    - os: osx
-      compiler: gcc
     # osx with xcode/clang (default)
     - os: osx
       compiler: clang