Update Travis CI configuration.

.travis.yml

@@ -30,12 +30,12 @@ addons:
     #   https://github.com/travis-ci/apt-source-whitelist/blob/master/ubuntu.json
     sources:
     - ubuntu-toolchain-r-test
-    - llvm-toolchain-trusty-5.0
+    - llvm-toolchain-trusty-6.0
     packages:
     - cmake
-    - gcc-7
-    - g++-7
-    - clang-5.0
+    - gcc-8
+    - g++-8
+    - clang-6.0
 
 install:
 # Travis doesn't have a DSL for installing homebrew packages yet. Status tracked
@@ -47,10 +47,10 @@ install:
 - if [ "$TRAVIS_OS_NAME" == "osx" ]; then
     brew update;
     if [ -L /usr/local/include/c++ ]; then rm /usr/local/include/c++; fi;
-    brew install gcc@7;
+    brew install gcc@8;
   fi
 # /usr/bin/gcc is stuck to old versions on both Linux and OSX.
-- if [ "$CXX" = "g++" ]; then export CXX="g++-7" CC="gcc-7"; fi
+- if [ "$CXX" = "g++" ]; then export CXX="g++-8" CC="gcc-8"; fi
 - echo ${CC}
 - echo ${CXX}
 - ${CXX} --version