travis: Retry flaky apt update at build start

.travis.yml

@@ -9,7 +9,9 @@ go_import_path: github.com/purpleidea/mgmt
 sudo: true
 dist: trusty
 before_install:
-  - if [[ "$TRAVIS_OS_NAME" != "osx" ]]; then sudo apt update; fi
+  # apt update tends to be flaky in travis, retry up to 3 times on failure
+  # https://docs.travis-ci.com/user/common-build-problems/#travis_retry
+  - if [[ "$TRAVIS_OS_NAME" != "osx" ]]; then travis_retry sudo apt update; fi
   - git config remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
   - git fetch --unshallow
 install: 'make deps'