Long labels wrongly positioned for vertical sliders

[ValentinH]

Steps to reproduce

1. Use quite long labels
2. Set orientation to vertical

Demo: http://jsfiddle.net/nqnwt57n/

Expected behaviour

Labels should be correctly positioned at the left of the slider

Actual behaviour

Labels starts from the right and are over the slider

Fix

Use left position value instead of right.

[dbartholomae]

Just created a PR for this: #306