Add gcc9, clang8 to Travis

.travis.yml

@@ -78,6 +78,16 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
 
+    - os: linux
+      compiler: g++-9
+      env: TOOLSET=gcc COMPILER=g++-9 CXXSTD=11,14,17,2a
+      addons:
+        apt:
+          packages:
+            - g++-9
+          sources:
+            - ubuntu-toolchain-r-test
+
     - os: linux
       compiler: g++-8
       env: UBSAN=1 TOOLSET=gcc COMPILER=g++-8 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
@@ -188,6 +198,17 @@ matrix:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-7
 
+    - os: linux
+      compiler: clang++-8
+      env: TOOLSET=clang COMPILER=clang++-8 CXXSTD=11,14,17,2a
+      addons:
+        apt:
+          packages:
+            - clang-8
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-trusty-8
+
     - os: linux
       compiler: clang++-6.0
       env: UBSAN=1 TOOLSET=clang COMPILER=clang++-6.0 CXXSTD=11,14,17,2a UBSAN_OPTIONS=print_stacktrace=1