Add test target for Clang 5.0 and C++14.

.travis.yml

@@ -10,6 +10,17 @@ cache:
   - ${TRAVIS_BUILD_DIR}/deps/doxygen
 matrix:
   include:
+  - compiler: clang
+    addons:
+      apt:
+        sources:
+        - ubuntu-toolchain-r-test
+        - llvm-toolchain-trusty-5.0
+        packages:
+        - clang++-5.0
+    env:
+    - COMPILER=5.0
+    - CLI_CXX_STD=14
   - compiler: clang
     addons:
       apt: