Improve Composer dependency versions on semantic versioned projects

To make phpbb more compatible when used with other / custom applications the versioning on dependencies which use semantic versioning should be improved.

Symfony is using semantic versioning since 2.3 so we could switch from current implementation (3.1.x branch)

composer.json

...

"symfony/console": "2.3.*",

...

to

composer.json

...

"symfony/console": "~2.3",

...

See: ~ operator for those don't knowing the operator

For standalone installations there is still the composer.lock file which makes sure when using `composer install` that only the tested components are loaded (symfony 2.3.*).

But when using phpbb combined with another application which relies on symfony ~2.7 for example composer would still be able to resolve the dependencies and phpbb should still function as expected since symfony 2.7 is backwards compatible to 2.3

This is not only 3.1.6 related but I didn't want to select all versions for an improvement.