The "Api::getRoles" doesn't return any roles

In Api.php:

```
public function getRoles($project_key)
{
    return $this->api(self::REQUEST_GET, sprintf('/rest/api/2/project/%s/roles', $project_key), array(), true);
}
```

roles should be fixed as role,
i.e. return $this->api(self::REQUEST_GET, sprintf('/rest/api/2/project/%s/role', $project_key), array(), true);
