Updated TravisCI.

.travis.yml

@@ -11,12 +11,12 @@ addons:
     sources:
     - ubuntu-toolchain-r-test
     packages:
-    - gcc-5
-    - g++-5
+    - gcc-8
+    - g++-8
     - clang
 
 script:
-  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then make test CXX="g++-5" CC="gcc-5"; fi
+  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then make test CXX="g++-8" CC="gcc-8"; fi
   - if [ "$TRAVIS_OS_NAME" == "osx" ]; then make test; fi
 
 branches: