Added support to Symfony 5

[gnugat]

Added support to:

• Symfony YAML 5
• Symfony Filesystem 5 (dev)
• Symfony PHPUnit bridge 5 (dev)

[gnugat]

This should address #131.
Not sure why #128 got closed, and from the look of it #130 isn't providing Symfony 5 support.

[gnugat]

Tests fail due to:

• PHP 7.2: use of PHPUnit 8 (tests pass with PHPUnit 7)
• PHP 5.5, 5.4: no longer supported by phpenv (used in Travis)

I recommend dropping older PHP versions, and perhaps upgrading tests to PHPUnit 8 (see #129 to solve both), and even maybe dropping support to older symfony versions (see #130).

[stof]

* PHP 5.5, 5.4: no longer supported by phpenv (used in Travis)

Actually, they can be supported by forcing dist: trusty (which was the default previously)

[gnugat]

Good news, I managed to fix the PHPUnit BC break (Symfony PHPUnit Bridge allows a SYMFONY_PHPUNIT_REMOVE_RETURN_TYPEHINT=1 environment variable to fix that issue).
Bad news, the dist: trusty doesn't seem to do the trick, although I'm not quite sure why...

[gnugat]

[adrienfr]

Hi, do you know when a release can be expected to enjoy the SF5 compatibility ? Thanks!