Travis: use stem backtrace signals with timelimit

Part of 30117.

.travis.yml

@@ -211,7 +211,7 @@ script:
   ## We run `make check` because that's what https://jenkins.torproject.org does.
   - if [[ "$DISTCHECK" == "" && "$TEST_STEM" == "" ]]; then make check; fi
   ## Diagnostic for bug 29437: kill stem if it hangs for 15 minutes
-  - if [[ "$TEST_STEM" != "" ]]; then timelimit -p -t 540 -T 30 make src/app/tor test-stem; fi
+  - if [[ "$TEST_STEM" != "" ]]; then timelimit -p -t 540 -s USR1 -T 30 -S ABRT make src/app/tor test-stem; fi
   - if [[ "$DISTCHECK" != "" && "$TEST_STEM" == "" ]]; then make distcheck DISTCHECK_CONFIGURE_FLAGS="$CONFIGURE_FLAGS"; fi
   ## If this build was one that produced coverage, upload it.
   - if [[ "$COVERAGE_OPTIONS" != "" ]]; then coveralls -b . --exclude src/test --exclude src/trunnel --gcov-options '\-p' || echo "Coverage failed"; fi