Don't freeze the version string

For some reason Bundler can't use the gem from git when the version string is frozen:

Updating https://github.com/jowl/bundesstrasse.git
There was a RuntimeError while loading bundesstrasse.gemspec:
can't modify frozen string from
<redacted>/bundler/gems/bundesstrasse-9f095c846139/bundesstrasse.gemspec:7:in `eval_gemspec'