(travis) Temporarily allow-fail all trusty-based builds

Until the fallout from https://www.traviscistatus.com/incidents/2p40l49r3yxd
( travis-ci/travis-ci#6439 ) is fixed...

.travis.yml

@@ -363,6 +363,10 @@ matrix:
   # which ones of the above can fail
   allow_failures:
 
+    # FIXME work around https://github.com/travis-ci/travis-ci/issues/6439
+    - perl: 5.18-extras
+    - perl: 5.24.0_thr_qm
+
     # these run with various dev snapshots - allowed to fail
     - perl: cperl-5.24.0_thr_qm
     - perl: cperl-master_thr