Fixed Issue #70: Problem list should not squeeze when current page have less results

[Issue #70](https://github.com/appirio-tech/arena-web/issues/70). To reproduce the issue you can replace app\js\controllers\practiceProblemListCtrl.js with [this](https://www.dropbox.com/s/2d1mgysx67iessm/practiceProblemListCtrl.js?dl=1).