adapt-contrib-slider: in RTL mode on iOS the number on the marker flips over

[moloko]

When the marker is moving from one step to another, the number on the marker flips over:

This may be down to my test using an LTR language that's been forced into RTL mode? Be good if someone can verify this!

Tested on iOS11 iPhone and iPad.

I have checked to see if adaptlearning/adapt-contrib-slider#119 fixes this but it doesn't.