Python 2.4 fix to allow later error to propagate

Yes, virtualenv no longer supports Python 2.4. However, there is some non-Python 2.4 early on that prevents the "this script requires Python 2.6 or greater" message from propagating to the user.

```
[someguy@fcknrh ~]$ python virtualenv.py py_virtual
  File "virtualenv.py", line 1578
    cp_or_ln = (os.symlink if symlink else copyfile)
                            ^
SyntaxError: invalid syntax
[someguy@fcknrh ~]$ vim virtualenv.py # Same edits as in this PR
[someguy@fcknrh ~]$ python virtualenv.py py_virtual
ERROR: None
ERROR: this script requires Python 2.6 or greater.
```