Update .travis.yml to use gcc-6

The patch is created by @take_cheese in #3872

.travis.yml

@@ -1,18 +1,27 @@
 language: c
 
-sudo: false
-
 matrix:
   include:
     - os: linux
-      sudo: 9000
+      sudo: false
+      dist: trusty
     - os: osx
-      osx_image: xcode7.1
 
 addons:
   apt:
+    sources:
+      - ubuntu-toolchain-r-test
     packages:
       - gperf
+      - gcc-6
+      - g++-6
 
-env: MRUBY_CONFIG=travis_config.rb
+env:
+    MRUBY_CONFIG=travis_config.rb
+before_install:
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; brew install gcc@6 ; fi
+  - export CC=gcc-6
+  - export CXX=g++-6
+  - export LD=gcc-6
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then export LDFLAGS="-fuse-ld=gold" ; fi    
 script: "./minirake all test"