Add `travis_retry` to apt-get operation

[deivid-rodriguez]

What was the end-user problem that led to this PR?

The problem was another failed build due to apt-get install failing.

What was your diagnosis of the problem?

My diagnosis was TravisCI networking is unstable at the moment.

What is your fix for the problem, implemented in this PR?

Not a permanent fix, since that doesn't depend on us, but using travis_retry should increase reliability.

[deivid-rodriguez]

@bundlerbot merge

[ghost]

Build succeeded

• continuous-integration/travis-ci/push