Make trainee maintainers three times as likely to be picked

Trainee maintainers count as reviewers, but should get more reviews than
most reviewers, as they need practice to become a maintainer. This makes
them three times as likely to be picked (compared to another reviewer;
compared to before, it's roughly twice as likely at the current numbers
for each).

Also, switch to `Array#sample` because I think it looks nicer.

danger/roulette/Dangerfile

@@ -31,13 +31,15 @@ MARKDOWN
 
 def spin(team, project, category)
   reviewers = team.select { |member| member.reviewer?(project, category) }
+  traintainers = team.select { |member| member.traintainer?(project, category) }
   maintainers = team.select { |member| member.maintainer?(project, category) }
 
   # TODO: filter out people who are currently not in the office
   # TODO: take CODEOWNERS into account?
 
-  reviewer = reviewers[rand(reviewers.size)]
-  maintainer = maintainers[rand(maintainers.size)]
+  # Make traintainers have triple the chance to be picked as a reviewer
+  reviewer = (reviewers + traintainers + traintainers).sample
+  maintainer = maintainers.sample
 
   "| #{helper.label_for_category(category)} | #{reviewer&.markdown_name} | #{maintainer&.markdown_name} |"
 end