travis: rabbitmq signature update

Fix comes from rabbitmq:

> On December 1st, 2018 (GMT) all repositories under the RabbitMQ account on PackageCloud
will be switched to use the new signing keys. RabbitMQ users who install packages
from PackageCloud must import the new signing keys before the migration.
The recommended way of doing that is by re-running PackageCloud setup
scripts (RabbitMQ, Erlang). Signing keys can be downloaded and imported directly
as well (RabbitMQ, Erlang). If the new keys are not imported, package installation
will start failing with a signature verification error.

Author: 0xc0170

.travis.yml

@@ -30,6 +30,8 @@ before_install:
   # Setup ppa to make sure arm-none-eabi-gcc is correct version
   - sudo add-apt-repository -y ppa:team-gcc-arm-embedded/ppa
   - sudo add-apt-repository -y ppa:deadsnakes/ppa
+  # import the new keys for rabbitmq (fix for https://github.com/ARMmbed/mbed-os/issues/8945)
+  - curl -s https://packagecloud.io/install/repositories/rabbitmq/rabbitmq-server/script.deb.sh | sudo bash
   # Loop until update succeeds (timeouts can occur)
   - travis_retry $(! sudo apt-get update 2>&1 |grep Failed)