Allow failures for Travis CI arm64 jobs

We are finding more flaky tests thanks to them, so for now we prefer to
avoid false positive over slowing down the build.

.travis.yml

@@ -5,6 +5,7 @@ jobs:
   allow_failures:
     - os: osx
     - stage: sanitizers
+    - arch: arm64
   fast_finish: true
   include:
     - &test-linux