Unfreeze the gemspec's version string.

Avoids a "can't modify frozen String" error under older versions of Rubygems.

```
$ gem --version
1.8.28
$ bundle exec rake test
There was a RuntimeError while loading xeroizer.gemspec: 
can't modify frozen String from
  /Users/fimmtiu/xeroizer/xeroizer.gemspec:8:in `block in <main>'
```