vanity gem don't track conversions

[EDIT]: added a github issue

some infos about versions:

ruby 2.2.2 rails 4.1.12 vanity (2.0.0.beta8) - from master branch on github

I followed the guide at http://ift.tt/Sin99n . I did all the steps, I'm using activerecord adapter to connect to the db. I implemented the following example in the guide:

Step 2: Define your first A/B test

This experiment goes in the file experiments/price_options.rb:

ab_test "Price options" do
  description "Mirror, mirror on the wall, who's the better price of all?"
  alternatives 19, 25, 29
  metrics :signups
end
If the experiment uses a metric as above (“signups”), there needs to be a corresponding ruby file for that metric, experiments/metrics/signups.rb.

metric "Signup (Activation)" do
  description "Measures how many people signed up for our awesome service."
end
Step 3: Present the different options to your users

<h2>Get started for only $<%= ab_test :price_options %> a month!</h2>
Step 4: Measure conversion

Conversions are created via the track! method. For example:

class SignupController < ApplicationController
  def signup
    @account = Account.new(params[:account])
    if @account.save
      track! :signups
      redirect_to @acccount
    else
      render action: :offer
    end
  end
end

The tracking of the metrics seems to work, while the 'conversion' field on vanity dashboard remains fixed at 0

no clue how to fix.

Thanks in advance