def get_installed_version(vcs):
    """Get the installed version for this project.

    Args:
        vcs (easyci.vcs.base.Vcs)

    Returns:
        str - version number

    Raises:
        VersionNotInstalledError
    """
    version_path = _get_version_path(vcs)
    if not os.path.exists(version_path):
        raise VersionNotInstalledError
    with open(version_path, 'r') as f:
        return f.read().strip()