<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def update(runas=None, path=None):
    '''
    Updates the current versions of rbenv and ruby-build

    runas
        The user under which to run rbenv. If not specified, then rbenv will be
        run as the user under which Salt is running.

    CLI Example:

    .. code-block:: bash

        salt '*' rbenv.update
    '''
    path = path or _rbenv_path(runas)
    path = os.path.expanduser(path)

    return _update_rbenv(path, runas) and _update_ruby_build(path, runas)