travis: switch from clang-3.8 to clang

As travis provides a relatively fresh clang by default nowadays,
it makes sense to use default clang in clang-based test builds.

* .travis.yml (matrix): Change clang-3.8 to clang.

.travis.yml

@@ -27,13 +27,13 @@ matrix:
     - compiler: gcc
       env:
         - TARGET=x86
-    - compiler: clang-3.8
+    - compiler: clang
       env:
         - TARGET=x86_64
     - compiler: musl-gcc
       env:
         - TARGET=x86_64
-    - compiler: clang-3.8
+    - compiler: clang
       env:
         - TARGET=x86
     - compiler: musl-gcc