[ci] minor, bring back the removed assertion

.ci/run-coveralls.sh

@@ -3,6 +3,8 @@
 set -x
 set -o errexit -o nounset
 
+if test x"$TRAVIS_REPO_SLUG" != x"harfbuzz/harfbuzz"; then exit; fi
+
 pip install --user nose
 pip install --user cpp-coveralls
 export PATH=$HOME/.local/bin:$PATH