[2.x] Add branch-alias

[dunglas]

To allow to use the ^2.8@dev notation.

[sanpii]

I don’t understand why #229 and #230 give two alias for the master branch.

Naively, I was write something like that:

    "extra": {
        "branch-alias": {
            "dev-2.x": "2.8.x-dev",
            "dev-master": "3.0.x-dev"
        }
    }

[sanpii]

Tests fail due to Behat/MinkExtension#310

[dunglas]

This one is buggy! Of course you’re right, it must point to the 2.x branch.

[sanpii]

If it’s possible, you can make one PR for master, I will cherry pick it to 2.x branch.

[dunglas]

IMO it should be 2 separate commit (it's useless to have this one in master).
In fact this commit looks ok (see https://github.com/symfony/symfony/blob/3.2/composer.json for instance).

[sanpii]

Ok, I follow your opinion, I don’t master branch aliases.