Travis: Added Clang 8

.travis.yml

@@ -97,6 +97,11 @@ matrix:
           sources: ['ubuntu-toolchain-r-test','llvm-toolchain-trusty-7']
           packages: ['clang-7']
       env: SCRIPT=test _CC=clang-7 _CXX=clang++-7
+    - addons:
+        apt:
+          sources: ['ubuntu-toolchain-r-test','llvm-toolchain-trusty-8']
+          packages: ['clang-8']
+      env: SCRIPT=test _CC=clang-8 _CXX=clang++-8
     - env: SCRIPT=coverage
     - os: osx
       osx_image: xcode7.3