It's 7 years later, but this is still is still coming up when I Googled for this exact problem.
You can make it work with a standard ActionView::Helpers::FormBuilder#select
and feed it the set of month+years by hand
<% the_months = (Date.new(2020,1,1)..Date.new(2020,12,31)).select { |d| d.day == 1 } %>
<%= f.select :start_date, the_months.map { |d| [d.strftime('%b %Y'), d] } %>