fix TypeError on multiplying sequence by non-int of type 'float' in Python 3

Error occurred with Python 3.3.3:

```
$ python3 pipstat.py smartypants
Fetching statistics for 'smartypants'. . .
Traceback (most recent call last):
  File "pipstat.py", line 152, in <module>
    main()
  File "pipstat.py", line 132, in main
    graph = bargraph(version_downloads)
  File "pipstat.py", line 48, in bargraph
    bar = (value / scale) * TICK
TypeError: can't multiply sequence by non-int of type 'float'
```