Add a gem source.

    :param source_uri: string
        The source URI to add.
    :param gem_bin: string : None
        Full path to ``gem`` binary to use.
    :param ruby: string : None
        If RVM or rbenv are installed, the ruby version and gemset to use.
        Ignored if ``gem_bin`` is specified.
    :param runas: string : None
        The user to run gem as.

    CLI Example:

    .. code-block:: bash

        salt '*' gem.sources_add http://rubygems.org/

def sources_add(source_uri, ruby=None, runas=None, gem_bin=None):
    '''
    Add a gem source.

    :param source_uri: string
        The source URI to add.
    :param gem_bin: string : None
        Full path to ``gem`` binary to use.
    :param ruby: string : None
        If RVM or rbenv are installed, the ruby version and gemset to use.
        Ignored if ``gem_bin`` is specified.
    :param runas: string : None
        The user to run gem as.

    CLI Example:

    .. code-block:: bash

        salt '*' gem.sources_add http://rubygems.org/
    '''
    return _gem(['sources', '--add', source_uri],
                ruby,
                gem_bin=gem_bin,
                runas=runas)