.travis(.yml): no longer need llvm-toolchain-trusty-7

clang-7 is enable by default

.travis.yml

@@ -12,10 +12,8 @@ addons:
   apt:
     sources:
     - ubuntu-toolchain-r-test
-    - llvm-toolchain-trusty-7
     packages:
     - g++-8
-    - clang-7
     - autoconf
     - automake
     - autotools-dev
@@ -35,7 +33,6 @@ addons:
 before_install:
   - $CC --version
   - if [ "$CXX" = "g++" ]; then export CXX="g++-8" CC="gcc-8"; fi
-  - if [ "$CXX" = "clang++" ]; then export CXX="clang++-7" CC="clang-7"; fi
   - $CC --version
   - go version
   - cmake --version