Add gcc-10 to Travis

.travis.yml

@@ -156,6 +156,28 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
 
+    - os: linux
+      dist: bionic
+      compiler: g++-10
+      env: TOOLSET=gcc COMPILER=g++-10 CXXSTD=03,11,14
+      addons:
+        apt:
+          packages:
+            - g++-10
+          sources:
+            - ubuntu-toolchain-r-test
+
+    - os: linux
+      dist: bionic
+      compiler: g++-10
+      env: TOOLSET=gcc COMPILER=g++-10 CXXSTD=17,2a
+      addons:
+        apt:
+          packages:
+            - g++-10
+          sources:
+            - ubuntu-toolchain-r-test
+
     - os: linux
       compiler: g++-7
       env: UBSAN=1 TOOLSET=gcc COMPILER=g++-7 CXXSTD=03,11,14,17 UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold