Small update to placeholder styling

Firefox 19+ has an opacity set to `::-moz-placeholder`, so it was never *really* `#aaa` to begin with.
This gets rid of the `!important` and the `:hover`, making it a lot more consistent with Chrome (colour-wise).

Doesn't seem to have any adverse affects on my end when changing it to this. It's pretty minor, so I apologise for that.