tris: allow failure of NSS interop tests at HEAD

.travis.yml

@@ -22,6 +22,7 @@ matrix:
   fast_finish: true
   allow_failures:
     - env: MODE=interop CLIENT=boring REVISION=origin/master
+    - env: MODE=interop CLIENT=tstclnt REVISION=default ZRTT=1
 
 install:
   - if [ "$MODE" = "interop" ]; then ./_dev/tris-localserver/start.sh -d && docker ps -a; fi