Remove default parameter of is_php()

It was simply pointless.

I know it's the bigger PHP 5 update, but why 5.3 in particular? Why not the minimum supported version, 5.2.4? For the record, previous value was 5.0. We could put 5.3.3, it has interesting fixes. Hey, let's put 5.4 as it's the lowest active branch. Ummm no, let's rather put the edgy 5.5.

And imagine this in code:

    // here we compare against some magical version which could change at any time.
    // and no, we won't tell which one it is, go find it yourself. ha ha.
    is_php();