`track_type` is not a mandatory field in JSON for track entity

A few days ago I found your CLI for dealing with SoundCloud API. It was a great luck for me, because I was going to write my own. 
Unfortunately, when I tried to get the info about this track https://soundcloud.com/relaxdaily/n082-light-beautiful-instrumental-music I got the following message "Unable to get track information."
I started my investigation and found that the `track_type` field in JSON was empty but your expected it shouldn't be empty... So, here is my PR.