[Card] Extra margins show when using ripple

When `matRipple` is added to a ``, the card expands on click.

Reproduction: https://stackblitz.com/edit/angular-1mherw?file=app/card-overview-example.html

This is due to the styling rule that says every element in the card except for the last should have bottom margin. When the ripple is added, the last element is no longer last and so it gains the margin bottom.

https://github.com/angular/material2/blob/master/src/lib/card/card.scss#L190

Start with the styling rule near src/lib/card/card.scss#L190 and use the linked StackBlitz reproduction to observe the card expansion when matRipple is added to mat-card. The fix is done when the card no longer gains an extra bottom margin or expands on click while retaining the existing card spacing behavior.

Written by the indexing model from the issue text.