travis: Attempt to cut down on flaky failures

Travis has been spuriously failing a LOT. Hopefully this reduces some of
those failures.

.travis.yml

@@ -9,6 +9,8 @@ go_import_path: github.com/purpleidea/mgmt
 sudo: true
 dist: trusty
 before_install:
+  # as per a number of comments online, this might mitigate some flaky fails...
+  - sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 0C49F3730359A14518585931BC711F9BA15703C6
   # apt update tends to be flaky in travis, retry up to 3 times on failure
   # https://docs.travis-ci.com/user/common-build-problems/#travis_retry
   - if [[ "$TRAVIS_OS_NAME" != "osx" ]]; then travis_retry travis_retry sudo apt update; fi