Hide next button when there is one slide only

[ghost]

This hide the next button as well as the prev button when there is one slide only!! Fixes issue https://github.com/adaptlearning/adapt-contrib-narrative/issues/151

[ghost]

This makes sense with the following lines: https://github.com/adaptlearning/adapt-contrib-narrative/blob/master/js/adapt-contrib-narrative.js#L234-L237

[moloko]

+1

[oliverfoster]

+1

[himanshu1618]

+1