Update travis build matrix

.travis.yml

@@ -1,20 +1,27 @@
 language: cpp
-dist: trusty
 
-compiler:
-  - clang
-  - gcc
-
-env:
-  matrix:
-    - ENABLE_UI=OFF
-    - ENABLE_UI=ON
-
-before_install:
-  - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
-  - sudo apt-get update -qq
-  - sudo apt-get install -y -qq g++-4.8
-  - if [ "$CXX" = "g++" ]; then export CXX="g++-4.8"; fi
+matrix:
+  include:
+    - os: osx
+      osx_image: xcode9
+      env:
+        - ENABLE_UI=ON
+    - os: linux
+      env:
+        - ENABLE_UI=OFF
+    - os: linux
+      env:
+        - ENABLE_UI=ON
+    - os: linux
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-7
+      env:
+        - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7"
+        - ENABLE_UI=ON
 
 before_script:
   - mkdir build