Allow to require deployer master through alias

Add `"elfet/deployer": "~2.1.0@dev"` to composer in order to fetch master branch.

This also would remove a warning when requiring dev-master and validating the composer.json:
``` console
$ composer validate
./composer.json is valid, but with a few warnings
See http://getcomposer.org/doc/04-schema.md for details on the schema
require.elfet/deployer : unbound version constraints (dev-master@dev) should be avoided
```