[travis] Only run coveralls if testing behdad/harfbuzz

Fixes failures in others' forks, like:
https://travis-ci.org/khaledhosny/harfbuzz/jobs/99719710#L3912

.travis.yml

@@ -28,7 +28,7 @@ script:
   - ./configure $CONFIGURE_OPTS
   - make
   - make check
-  - if [ "$TRAVIS_OS_NAME" == "linux" -a "$CC" == "gcc" ]; then rm -f src/.libs/NONE.gcov; touch src/NONE; coveralls -e docs; fi
+  - if [ "$TRAVIS_OS_NAME" == "linux" -a "$CC" == "gcc" -a "$TRAVIS_SLUG" == "behdad/harfbuzz" ]; then rm -f src/.libs/NONE.gcov; touch src/NONE; coveralls -e docs; fi
 after_success:
   - if [ "$TRAVIS_OS_NAME" == "linux" -a "$CC" == "gcc" -a "$TRAVIS_SECURE_ENV_VARS" == "true" ]; then bash .ci/deploy-docs.sh; fi
 notifications: