Fix Travis failing tests

[javiereguiluz]

This is our Travis CI test suite now:

The failing tests look like this:

Full details: https://travis-ci.org/symfony/demo/jobs/543634635

This is NOT related to #997. It was failing way before. Anyone can help us fix this? Thanks!

[dmaicher]

Currently looking into this. Can reproduce it locally using php 7.1.30

[dmaicher]

@javiereguiluz fixed in #1001