Add g++ 8 to Travis

.travis.yml

@@ -84,6 +84,16 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
 
+    - os: linux
+      compiler: g++-8
+      env: TOOLSET=gcc COMPILER=g++-8 CXXSTD=11,14,17
+      addons:
+        apt:
+          packages:
+            - g++-8
+          sources:
+            - ubuntu-toolchain-r-test
+
     - os: linux
       compiler: g++
       env: NVCC=1