Use Travis CI's retry feature

[nfischer]

Our builds are very flaky. We may be able to reduce this flakiness (and the need for manual restarts) using Travis's retry feature.

Before we blindly add travis_retry, we should figure out which steps are most likely to fail (and would benefit from a retry).

Documentation: https://docs.travis-ci.com/user/common-build-problems/#travis_retry
Example PR: https://github.com/vaadin/vaadin-grid/pull/153/files