Split g++ 8 Travis job, because log length

.travis.yml

@@ -107,7 +107,17 @@ matrix:
 
     - os: linux
       compiler: g++-8
-      env: TOOLSET=gcc COMPILER=g++-8 CXXSTD=03,11,14,17,2a
+      env: TOOLSET=gcc COMPILER=g++-8 CXXSTD=03,11,14
+      addons:
+        apt:
+          packages:
+            - g++-8
+          sources:
+            - ubuntu-toolchain-r-test
+
+    - os: linux
+      compiler: g++-8
+      env: TOOLSET=gcc COMPILER=g++-8 CXXSTD=17,2a
       addons:
         apt:
           packages: