Sass lint: add missing leading-zero

Use a leading zero for numbers.

Pro:
- css-guidelines
(https://github.com/chris-pearce/css-guidelines#formatting)
- a mathemacial point of view (http://hey.georgie.nu/leadingzero/).

Cons:
- Codestyle.co (http://codestyle.co/Guidelines/css#Syntax)
- Less concise