spec: bump 389-ds-base to 1.3.7.6-1
    
    To avoid insidious bug during server installation on Fedora 27,
    the dependency of 389-ds-base is bumped.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1488295
    
    Signed-off-by: Tomas Krizek <tkrizek@redhat.com>
    Reviewed-By: Thierry Bordaz <tbordaz@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>