Do not use clang for coverage

When using clang, this will create a race condition between
clang/gcc builds. Coveralls will take the first one. As GCC and clang
are counting line coverage differently, coveralls regularly fails.

.travis.yml

@@ -35,5 +35,3 @@ matrix:
       after_success:
         - coveralls
     - compiler: clang
-      after_success:
-        - coveralls --gcov llvm-cov --gcov-options gcov