If i look into composer.json (even in piwik 3 alpha) i can read e.g.
"symfony/console": "~2.6.0",
…
"symfony/var-dumper": "~2.6.0",
"symfony/yaml": "~2.6.0"
This way the piwik can use only 2.6 branch, which is outdated and not supported for more then year now (The Release Process (Symfony Docs))
Much better would be to define it without the .0, so that it could be updated to latest version 2 branch (2.8) which is fully backwards compatible and officialy supported for another years. So it should be
“symfony/console”: "~2.8"
“symfony/var-dumper”: “~2.8”,
“symfony/yaml”: “~2.8”
And for piwik alpha 3 branch you should use symfony 3 branch.
And of course the same for the other packages.