Revert "Update .travis.yml to use gcc-6"

This change caused Travis CI errors on OSX.  This reverts
commit c76631d445.

.travis.yml

@@ -1,27 +1,20 @@
 language: c
 
+sudo: false
+
 matrix:
   include:
     - os: linux
-      sudo: false
-      dist: trusty
+      sudo: 9000
     - 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
-before_install:
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; brew install gcc@6 || breq link --overwrite gcc@6 ; fi
-  - export CC=gcc-6
-  - export CXX=g++-6
-  - export LD=gcc-6
-  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then export LDFLAGS="-fuse-ld=gold" ; fi    
+env: MRUBY_CONFIG=travis_config.rb
 script: "./minirake all test"