Add missing '_display_code_snippet' variable

[ianfp]

This is required for compatibility with Symfony 3.3.9 (and earlier?). See

symfony/symfony#24265

[sonnylloyd]

This solution fixed the issue stated

[kopaygorodsky]

Would be nice to have this fix in master.

[kick-the-bucket]

About the failing build:
This bundle states that it supports PHP 5.5.0 || ^7.0, but strangely has a composer.lock commited with a lot of dependencies, that need at least 5.5.9 or even 5.6.
For a library trying to support different platforms and package versions the composer.lock file should either be omitted at all or generated using the lowest PHP version supported.