add alias for master branch

A bit of explanation on this:

- Version key is not required. I don´t remember why I put there... maybe it was something with an old composer version... but anyway it is not required anymore

- Master branch should be aliased as 1.0.x-dev and whenever you bump to 1.1, to 1.1.x-dev

- When you tag a version, you should add something like

    "extra": {
        "branch-alias": {
            "v1.0.a.6": "1.0.6"
        }
    }

If you could retag 1.0.a.6 that would be great, otherwise, please keep this in mind for the next 1.0.7 (I don´t know if you had already any plans for it)

and meanwhile, master branch should  remain aliased as 1.0.x-dev or similar