fixed `ZeroDivisionError: division by zero` in tags_cloud.html

I used `tinker --build` command with enabled tags cloud, but was throwing errors below.

```
$ tinker -b
...
...
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [ 16%] 2013/11/19/something
Exception occurred:
  File "/python2.7/site-packages/tinkerer/themes/boilerplate/tags_cloud.html", line 22, in top-level template code
    {%- set size = (fontsize_max-fontsize_min)*(x-count_min)/(count_max-count_min) + fontsize_min %}
ZeroDivisionError: division by zero
The full traceback has been saved in /var/folders/yc/2zd6v8td575bnv20qb608nd40000gn/T/sphinx-err-O5eNRN.log, if you want to report the issue to the developers.
```