Update _offcanvas.scss

"height:100%;" is superfluous since the class has "position: absolute;", "top:0;" and "bottom:0;"
This can also create issues when viewing the off canvas menu on a device with a screen size width <= 640.

See:
![screenshot 2013-12-30 02 07 27](https://f.cloud.github.com/assets/6283320/1820523/008a4be8-70f0-11e3-8729-e7d6e1256bcd.png)

![screenshot 2013-12-30 02 07 46](https://f.cloud.github.com/assets/6283320/1820524/048c581c-70f0-11e3-8621-5c7b08e1ac64.png)