fixup gemspec

I keep getting
```
Using foreman_dhcp_browser (0.0.1) from https://github.com/theforeman/foreman_dhcp_browser.git (at master)
foreman_dhcp_browser at /usr/share/foreman/vendor/ruby/1.9.1/bundler/gems/foreman_dhcp_browser-f5e0c5c6110d did not have a valid gemspec.
This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
The validation message from Rubygems was:
  "FIXME" or "TODO" is not a description
```