Restore PHPUnit test runs

[danielbachhuber]

Fixes #150

[danielbachhuber]

@schlessera Is a branch-alias appropriate for fixing this error?

wp-cli/wp-cli[v2.6.0, ..., v2.7.1] require wp-cli/php-cli-tools ~0.11.2 -> satisfiable by wp-cli/php-cli-tools[v0.11.2, ..., v0.11.16] from composer repo (https://repo.packagist.org/) but wp-cli/php-cli-tools is the root package and cannot be modified. See https://getcomposer.org/dep-on-root for details and assistance.

If so, what should it be?

[schlessera]

@danielbachhuber Yes, a branch alias together with a stability adjustment in this package should solve the above issue. It should be something like this:

{
    "extra": {
        "branch-alias": {
            "dev-master": "0.11.x-dev"
        }
    }
    "minimum-stability": "dev",
    "prefer-stable": true
}