Travis: Try to set clang version explicitly

.travis.yml

@@ -41,10 +41,11 @@ addons:
       - openssl
       - libssl-dev
 #      - gcc-5
-#      - clang-3.8
+      - clang-3.8
     sources:
       - kubuntu-backports
-#     - ubuntu-toolchain-r-test
+      - llvm-toolchain-precise-3.8
+
 
 before_install:
   - if [ "${TRAVIS_OS_NAME}" == "linux" ]; then