executable files: don't assume python2.5.

executable files: don't assume python2.5.

The forcing of version 2.5 was leftover from before, when it was
accidentally selecting python 2.4 by accident on some distros when both
versions are installed.  But actually that's fine; bup works in python 2.4
without problems.

So let's not cause potentially *more* portability problems by forcing python
2.5 when it might not exist.