Fix for Safari and input:focus

See [this issue](https://github.com/callemall/material-ui/issues/112) and [this patch](https://github.com/callemall/material-ui/pull/114). I'd just go with the general sibling selector for the whole [_input.scss](https://github.com/gpbl/material-ui-sass/blob/master/material-ui/components/_input.scss) file, so to avoid such deep nesting.
