Clang 5.0 CI build added and Clang 3.8 removed.

.travis.yml

             - g++-7
           sources: &sources
             - ubuntu-toolchain-r-test
-            - llvm-toolchain-precise
-            - llvm-toolchain-precise-3.9
-            - llvm-toolchain-precise-3.8
-            - sourceline: 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-4.0 main'
-              key_url: 'http://apt.llvm.org/llvm-snapshot.gpg.key'
+            - llvm-toolchain-trusty
+            - llvm-toolchain-trusty-3.9
+            - llvm-toolchain-trusty-4.0
+            - llvm-toolchain-trusty-5.0
     - env: CXX=g++-6 CC=gcc-6
       addons:
         apt:
           packages:
             - g++-5
           sources: *sources
-    - env: CXX=clang++-4.0 CC=clang-4.0
+    - env: CXX=clang++-5.0 CC=clang-5.0
       addons:
         apt:
           packages:
-            - clang-4.0
+            - clang-5.0
             - libc++-dev
           sources: *sources
-    - env: CXX=clang++-3.9 CC=clang-3.9
+    - env: CXX=clang++-4.0 CC=clang-4.0
       addons:
         apt:
           packages:
-            - clang-3.9
+            - clang-4.0
             - libc++-dev
           sources: *sources
-    - env: CXX=clang++-3.8 CC=clang-3.8
+    - env: CXX=clang++-3.9 CC=clang-3.9
       addons:
         apt:
           packages:
-            - clang-3.8
+            - clang-3.9
             - libc++-dev
           sources: *sources