fix version string

Bundler complains about the version format.

```Updating git://github.com/mboeh/woodhouse.git There was a ArgumentError while loading woodhouse.gemspec: Malformed version number string 2.0.0-alpha1 from /opt/boxen/rbenv/versions/2.0.0-p451/lib/ruby/gems/2.0.0/bundler/gems/woodhouse-f875fa03ba1c/woodhouse.gemspec:7:in `block in <main>'```