Improved compatibility with slightly older versions of Firefox.

However before Firefox 22 you still have to manually enable flexbox. But now
there is a warning, telling the user to do so.

Users with older versions than Firefox 18, get a warning to update their
browser instead, because of flexbox isn't available at all.
Replaced 'keyup' with 'input' event.

That way the search is executed not only when a key is pressed while an input
field has the focus, but also when input was provided by an other way, like
pasting with the mouse.

Also the 'keyup' event isn't fired in Fennec and therefore the cocktail
search was entirely broken in Fennec, before.