travis-ci: do not run coveralls with clang (SIGSEGV)

This will fix coveralls badge

See: https://travis-ci.org/libevent/libevent/jobs/458590276#L1425
(cherry picked from commit 3e6bec7c1aa416e3e6e85c7bda1bdc98b778d23c)

.travis.yml

@@ -39,6 +39,10 @@ compiler:
   - clang
 
 before_install:
+  # do not run with clang, since it fails (SIGSEGV)
+  - if [ "$CC" = "clang" ]; then
+      unset COVERALLS;
+    fi
   - if [ -n "$COVERALLS" ]; then
       pip install --user cpp-coveralls;
     fi