Update to gcc-8 and clang-8.0

Author: Justus H

.travis.yml

@@ -10,24 +10,24 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
           packages:
-            - g++-5
+            - g++-8
             - zip
       env: 
-        ENV_CXX=g++-5 
-        ENV_CC=gcc-5 
+        ENV_CXX=g++-8 
+        ENV_CC=gcc-8
         NIGHTLY_UPLOAD=1
     - compiler: clang
       addons:
         apt:
           sources:
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-xenial-6.0
+            - llvm-toolchain-xenial-8.0
           packages:
-            - clang-6.0
+            - clang-8.0
             - zip
       env: 
-        ENV_CXX=clang++-6.0 
-        ENV_CC=clang-6.0 
+        ENV_CXX=clang++-8.0 
+        ENV_CC=clang-8.0 
         NIGHTLY_UPLOAD=0
 
 before_script: