Clang 11 CI build added (#173).

.travis.yml

@@ -16,6 +16,7 @@ jobs:
     - env: COMPILER=gcc-9
     - env: COMPILER=gcc-8
     - env: COMPILER=gcc-7
+    - env: COMPILER=clang-11
     - env: COMPILER=clang-10
     - env: COMPILER=clang-9
     - env: COMPILER=clang-8