.travis.yml: Adding missing envs and compiler types to macOS builds.

.travis.yml

@@ -62,13 +62,18 @@ matrix:
       compiler: clang
     # osx with gcc6
     - os: osx
+      compiler: gcc
       env:
         - MATRIX_EVAL="brew install gcc6 && CC=gcc-6 && CXX=g++-7"
     # osx with gcc7
     - os: osx
+      compiler: gcc
+      env:
         - MATRIX_EVAL="brew install gcc7 && CC=gcc-7 && CXX=g++-7"
     # osx with gcc8
     - os: osx
+      compiler: gcc
+      env:
         - MATRIX_EVAL="brew install gcc8 && CC=gcc-8 && CXX=g++-8"
     # osx with xcode8.3/clang
     - os: osx