Uncaught promise error in 1.1.0

Hi,

Firstly, I want to thank you for your library, it adds a nice touch to our application.
As I am a developer, it's sometime nice when we know how our creation is used, so I've attached a partial screenshot to show to you our use case.

Secondly, I encouters a little issue since I've upgrade progressbar.js from 1.0.1 to 1.1.0.
In my browser console, there is sometime one or multiple uncaught promise errors:

```
Uncaught (in promise) {offset: 289.0670000019968}
Promise.then (async)
animate @ progressbar.js:1183
animate @ progressbar.js:1406
```

It does'nt seem to affect the UI. Unfortunatly, I have not been able to find exactly why it occurs.

By by looking into the diffs from [1.0.1 to 1.1.0](https://github.com/kimmobrunfeldt/progressbar.js/compare/1.0.1...1.1.0#diff-4d74f839e0b8f64858c424ed76e895c1R118-R121),
I have been able to find that the error may be caused since the upgrade of the `shifty` dependency from the [Pull Request #196](https://github.com/kimmobrunfeldt/progressbar.js/pull/196) ([commit](https://github.com/kimmobrunfeldt/progressbar.js/commit/cebeb0786a331de9e2083e416d191c5181047e53)).

By adding an empty catch function to [R118-R121](https://github.com/kimmobrunfeldt/progressbar.js/compare/1.0.1...1.1.0#diff-4d74f839e0b8f64858c424ed76e895c1R118-R121), the promise error is handled and it seem to not affect the UI. But, I'm not sure that it is the right way to fix this problem...

```js
.catch(function(state) {})
```

I hope it helps you, thank you very much!

---------------------------


Start in progressbar.js at lines 118-121 and follow the animate paths shown at lines 1183 and 1406. Compare versions 1.0.1 and 1.1.0, including the shifty upgrade from pull request #196, and reproduce the promise rejection. Done means the uncaught promise error no longer appears without changing the unaffected UI behavior.

Written by the indexing model from the issue text.