Replace "overflow: scroll;" with "overflow: auto;"

"overflow: scroll;" always* shows the scroll bars, even if they are not
needed; if the scroll bars should be shown only when needed "overflow:
auto;" should be used instead.

*In newer browsers "overflow: scroll;" behaves like "overflow: auto;",
so this change makes no difference in those cases, but older browsers
respect the expected behaviour.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>