Use bundler instead of juwelier for cutting gems

Converted from using the juwelier gem to using bundler for cutting the gem. This helps make it more standard on what files to update (gitlab_kramdown.gemspec, lib/gitlab_kramdown/version.rb). It also allows someone to use bundler console or bundler exec console to load up a console in the environment of the gem, making it easier to play with it.

Rough instructions releasing the gem

• update lib/gitlab_kramdown/version.rb with the new version and commit (say to 0.9.0)
• rake release. This will
  • build the gem package to pkg/gitlab_kramdown-0.9.0.gem
  • add and push a tag like v0.9.0 to the repo
  • push commits to the repo
  • push the .gem package to Rubygems.org

If rake release hangs, you may not have entered the RubyGems credentials before. Kill the rake release and push the gem manually, like gem push pkg/gitlab_kramdown-0.9.0.gem and you will be prompted to enter the credentials.