Use twig.strict_variables: '%kernel.debug%'

[Nyholm]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Related tickets
Documentation
License	MIT

What's in this PR?

This will fix the following deprecation notice:

 8x: Relying on the default value ("false") of the "twig.strict_variables" configuration option is deprecated since Symfony 4.1. You should use "%kernel.debug%" explicitly instead, which will be the new default in 5.0.
    2x in DiscoveredClientsTest::testDiscoveredClient from Http\HttplugBundle\Tests\Functional
    2x in DiscoveredClientsTest::testDiscoveredClientWithProfilingEnabled from Http\HttplugBundle\Tests\Functional
    2x in DiscoveredClientsTest::testDisabledDiscovery from Http\HttplugBundle\Tests\Functional
    2x in DiscoveredClientsTest::testForcedDiscovery from Http\HttplugBundle\Tests\Functional

[Nyholm]

Thank you for the review. Travis is failing but they are unrelated.

[dbu]

thanks!

[Nyholm]

Thank you for the reviews and merging