Travis CI: Drop installation of Clang 4

.. as trusty images come with Clang 5 out of the box by now.
diff --git a/.travis.yml b/.travis.yml
index d6f40e2..4ac808c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -21,10 +21,7 @@
 
 addons:
   apt:
-    sources:
-      - llvm-toolchain-trusty-4.0
     packages:
-      - clang-4.0
       - cmake
       - docbook2x
       - dos2unix