fix (slider) - fixing slider default value in ng-repeat

md-slider does not work within ng-repeat.
when i have used md-slider without ng-repeat the slider sets on its proper position and aria-valuenow is also set with its proper value, but when i have used md-slider within ng-repeat the aria-valuenow="NaN" and slider is not set with its proper position.

Plunker demo of problem - http://plnkr.co/edit/PslB2p?p=preview

With this pull request the default value of md-slider in ng-repeat will be correct.