update vagrant minor version in readme

I was using vagrant 1.5.1, and I kept getting errors that was fixed in the workaround posted a few weeks ago chulkilee@c5d6e55

When upgrading to the new steps defined in this README, I kept getting

Bundler could not find compatible versions for gem "celluloid":
  In Gemfile:
    vagrant-berkshelf (>= 0) ruby depends on
      celluloid (~> 0.16.0.pre) ruby

    vagrant (= 1.5.1) ruby depends on
      celluloid (0.15.2)

When upgrading to vagrant 1.5.2 it works fine. Please accept this pull request to prevent people like me from wasting time :)