Since there is no Justin Android App for aural training I have been look for others. The one I'm trying out is called Perfect Ear Pro
It is only a US$1 and allows you to set up your own interval training programs.
What are other people using?
I use GNU Solfege, an open source program with lots of tests, including melodic intervals like in the lesson.
For anyone finding differentiating perfect 4ths and 5ths difficult, even with reference songs in mind, I've discovered a useful trick. Notice that a perfect 4th sounds resolved, whereas a perfect 5th sounds unresolved. So if you were to play the first two notes of Auld Lang Syne (a perfect 4th) it'll sound finished and complete, but play the first couple of notes of Star Wars (a perfect 5th) and it'll sound incomplete, like it's begging for more notes to be played to finish the tune off. Therefore if the interval sounds "finished" then it's a perfect 4th and if it sounds "unfinished" then it's a perfect 5th.
I've found this very helpful, particularly with notes in the low and high pitch ranges, where it's more difficult to just use reference songs.