I think the recommended way is to use a limited dependency, like ^1.5, and then tell Composer what the current dev-master actually represents with a branch-alias in wp-cli/wp-cli:

    "extra": {
        "branch-alias": {
            "dev-master": "1.5.x-dev"
        }
    }

This way, the dev-master branch satisfies the ^1.5 requirement as well, provided that development versions are allowed.

Thanks, will do once packagist.org comes back online - seems to be having issues today...