Ripple container is a square during animation time

(4 times slowed down ripple effect speed)

> What MDL Version are you using?

The one what is provided on getmdl.io : version v1.2.1

> What browser(s) is this bug affecting?

Google Chrome 54.0.2840.71 m (64-bit)
Opera 41.0.2353.46
Vivaldi 1.4.589.11 (Stable channel) (32-Bit)
Firefox 49.0.2

> **NOT** affected browsers:

Microsoft Edge 38.14393.0.0
Internet Explorer 11

> What OS (and version) are you using?

Windows 10 1607

> What are the steps to reproduce the bug?
1. Go to https://getmdl.io/components/index.html#buttons-section
2. Click and hold a button.

> What is the expected behavior?

Always visible border-radius

> What is the actual behavior?

While ripple is animating, the bounds of `.mdl-button__ripple-container` are square.
The `border-radius` of parent `.mdl-button` is ignored.

> Any other information you believe would be useful?

Adding `border-radius: inherit;` (or 2px) to `.mdl-button__ripple-container` will fix it.
https://github.com/google/material-design-lite/blob/88872e672e41c56af0a78a35b34373b8c4a8c49d/src/button/_button.scss#L224