Add Clang 8 to Travis testing

.travis.yml

@@ -112,17 +112,7 @@ matrix:
 
     - os: linux
       compiler: g++-8
-      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
+      env: TOOLSET=gcc COMPILER=g++-8 CXXSTD=03,11,14,17,2a
       addons:
         apt:
           packages:
@@ -252,6 +242,17 @@ matrix:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-7
 
+    - os: linux
+      compiler: clang++-8
+      env: TOOLSET=clang COMPILER=clang++-8 STANDARD=03,11,14,17,2a
+      addons:
+        apt:
+          packages:
+            - clang-8
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-trusty-8
+
     - os: linux
       compiler: clang++-libc++
       env: TOOLSET=clang COMPILER=clang++-libc++ CXXSTD=03,11,14,1z