Added wkhtmltopdf and wkhtmltoimage to default.sls for install onto serv...

...er
Yes, as expected, ``wkhtmltoimage`` is not a valid package:

    [33.33.33.10] out:   Name: wkhtmltopdf - Function: pkg.installed - Result: Changed
    [33.33.33.10] out: ----------
    [33.33.33.10] out:           ID: wkhtmltoimage
    [33.33.33.10] out:     Function: pkg.installed
    [33.33.33.10] out:       Result: False
    [33.33.33.10] out:      Comment: The following packages failed to install/update: wkhtmltoimage.
    [33.33.33.10] out:      Changes: