Checkbox Ripple position

Seems like Checkbox Ripple container aligned `1px` lower than it supposed to be.

[Next](https://github.com/google/material-design-lite/blob/master/src/checkbox/_checkbox.scss#L154) code compiles to `6px` instead of `7px`.

``` sass
top: -(($checkbox-ripple-size - $checkbox-label-height) / 2);
```

Maybe, [this](https://github.com/google/material-design-lite/blob/master/src/_variables.scss#L421) section of [_variables.scss](https://github.com/google/material-design-lite/blob/master/src/_variables.scss) should be corrected.