Fixes undefined method error when using progress_bar in Rails 4.

Not sure why, but after upgrading to Rails 4 this issue started. Now it explicitly loads the Time class form the default module to fix this issue, figured this change wouldn't hurt anyway.

```
undefined method `at' for ProgressBar::Time:Class
progress_bar-1.0.0/lib/progress_bar.rb:18:in `initialize'
```