material spec: vertical button text alignment

If you look closer at the button you will see that the text content a little aligned to the top of the button. It's a bit annoying. The reason why buttons behave in this way is because MDL uses line-height value of 36px - the height of button itself. So, if you change line-height, for example, to 1px - it fixes the issue:

``` sass
line-height: $button-height; // I beg you fix me!
```

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originally from #1864