Bugfix: ETA works again, when initializeing with a non zero starting_at.

Otherwise a "FloatDomainError Infinity" error was raised:

```ruby
> ProgressBar.create({starting_at: 0, format: '%e'})
=> #<ProgressBar:0/100>
> ProgressBar.create({starting_at: 1, format: '%e'})
FloatDomainError: Infinity
from [...]ruby/gems/2.1.0/gems/ruby-progressbar-1.7.0/lib/ruby-progressbar/components/time.rb:90:in `round`
```

This PR fixes the issue. Seems to be broken since ed6317453782e510c4d0644e20cc76b10c572e0d .