Require Vagrant 1.4.3
Fix #23; remove Gemfile.lock before installing gems

This doesn't make any sense to me. The file doesn't change, and gem
creates it itself. So why does "gem install" succeed if it isn't there?

Aliens.
Switch from Maruku to Kramdown

Maruku was throwing a bunch of warnings.

    Maruku#to_s is deprecated and will be removed or changed in a near-future version of Maruku.
Add a comment for the source directory

I don't want to override it because GitHub recommends against that. So
I'll just leave a comment there to remind myself.