Clang CI build removed as the lack of C++17 support on Travis CI (#87).

.travis.yml

@@ -21,17 +21,6 @@ matrix:
             - g++-7
           sources: &sources
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-trusty
-            - llvm-toolchain-trusty-3.9
-            - llvm-toolchain-trusty-4.0
-            - llvm-toolchain-trusty-5.0
-    - env: CXX=clang++-5.0 CC=clang-5.0
-      addons:
-        apt:
-          packages:
-            - clang-5.0
-            - libc++-dev
-          sources: *sources
 
 
 install: