Improved travisCI support

.travis.yml

@@ -1,11 +1,8 @@
 language: cpp
 
-os:
-  - linux
-  - osx
-
 matrix:
   include:
+    - os: linux
       - compiler: gcc
         addons:
           apt:
@@ -13,7 +10,8 @@ matrix:
               - ubuntu-toolchain-r-test
             packages:
               - g++-4.9
-      env: COMPILER=g++-4.9
+        env:
+          - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
       - compiler: gcc
         addons:
           apt:
@@ -21,7 +19,9 @@ matrix:
               - ubuntu-toolchain-r-test
             packages:
               - g++-7
-      env: COMPILER=g++-7
+        env:
+          - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
+    - os: osx
       - compiler: clang
         addons:
           apt:
@@ -42,7 +42,8 @@ matrix:
         env:
           - MATRIX_EVAL="CC=clang-5.0 && CXX=clang++-5.0"
 
-before_script: 
+before_install:
+    - eval "${MATRIX_EVAL}"
 
 script:
 - cmake --version