Merge pull request #119 from Rechi/ci

[ci][travis] use newer ubuntu & xcode versions

Author: Rechi

.travis.yml

@@ -10,17 +10,17 @@ language: cpp
 matrix:
   include:
     - os: linux
-      dist: trusty    
+      dist: xenial
       sudo: required
       compiler: gcc
     - os: linux
-      dist: trusty    
+      dist: xenial
       sudo: required
       compiler: clang
     - os: osx
-      osx_image: xcode7.3
+      osx_image: xcode9
     - os: osx
-      osx_image: xcode6.1
+      osx_image: xcode9.4
 
 #
 # The addon source is automatically checked out in $TRAVIS_BUILD_DIR,