improve uniqueness of conference urls

* make new conferences use more unique urls
  when being creating
* backward compatible for existing conferences
  to still use legacy less-unique urls
* better handle errors when a conference
  exists and has a url naming collision
* update gemfile for newer format
* bump version
revert change to .gitignore
Merge pull request #2 from brainlid/master

improve unique URL generation while maintaining backward compatibility