Prepare to release walker as a gem

- Rename gemspec correctly (it is `Vcloud::Walker` not `VcloudWalker`: http://guides.rubygems.org/name-your-gem/)
- Tidy gemspec
- Remove Gemfile.lock before testing
- The default rake task is spec so no need to say `bundle exec rake spec` in jenkins.sh
Merge pull request #54 from alphagov/prep-to-release-gem

[#62889568] Prepare to release walker as a gem