[travis] forgot to also re-enable clang installing

.travis.yml

@@ -16,9 +16,9 @@ addons:
     - g++-4.9
     - gcc-5
     - g++-5
-    #- clang-3.7
-    #- clang-format-3.7
-    #- clang-modernize-3.7
+    - clang-3.7
+    - clang-format-3.7
+    - clang-modernize-3.7
     - cmake
     - cmake-data
     - doxygen