concourse: test most-recent two rubies

because some things don't work on 2.4 but work on 2.3

[skip ci]

concourse/mini_portile.yml

@@ -1,15 +1,17 @@
-% ruby_version = RUBIES[:mri].last
-
 groups:
   - name: master
     jobs:
+      <% RUBIES[:mri].last(2).each do |ruby_version| %>
       - ruby-<%= ruby_version %>
+      <% end %>
+      <% RUBIES[:windows].last(2).each do |ruby_version| %>
       - win-ruby-<%= ruby_version %>-devkit
+      <% end %>
 
   - name: PRs
     jobs:
       - pr-pending
-      - ruby-<%= ruby_version %>-pr
+      - ruby-<%= RUBIES[:mri].last %>-pr
       - pr-success
 
 resource_types:
@@ -46,6 +48,7 @@ jobs:
   #
   #  master
   #
+  <% RUBIES[:mri].last(2).each do |ruby_version| %>
   - name: ruby-<%= ruby_version %>
     public: true
     plan:
@@ -63,7 +66,9 @@ jobs:
             - name: mini_portile
           run:
             path: ci/concourse/tasks/rake-test/task.sh
+  <% end %>
 
+  <% RUBIES[:windows].last(2).each do |ruby_version| %>
   - name: win-ruby-<%= ruby_version %>-devkit
     public: true
     plan:
@@ -81,6 +86,7 @@ jobs:
           run:
             path: powershell
             args: ["-File", "ci/concourse/tasks/rake-test/task.ps1"]
+  <% end %>
 
   #
   #  PRs
@@ -95,6 +101,7 @@ jobs:
       - put: mini_portile-pr
         params: {path: mini_portile-pr, status: pending}
 
+% ruby_version = RUBIES[:mri].last
   - name: ruby-<%= ruby_version %>-pr
     public: true
     serial_groups: [pr]