Revert to ruby 2.6.6 but add concurrent-ruby

I was getting this error:
zendesk/ruby-kafka#840

The heroku-kafka-demo-ruby app. also uses 
activesupport, which avoids this problem.

Author: digitalronin

Gemfile

@@ -1,11 +1,12 @@
 # frozen_string_literal: true
 
-ruby "2.5.8"
+ruby "2.6.6"
 
 source "https://rubygems.org"
 
 git_source(:github) { |repo_name| "https://github.com/#{repo_name}" }
 
-gem 'ruby-kafka'
+gem "concurrent-ruby"
+gem "ruby-kafka"
 gem "sinatra"
-gem 'puma'
+gem "puma"

Gemfile.lock

@@ -1,6 +1,7 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    concurrent-ruby (1.1.6)
     digest-crc (0.6.1)
       rake (~> 13.0)
     mustermann (1.1.1)
@@ -26,12 +27,13 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  concurrent-ruby
   puma
   ruby-kafka
   sinatra
 
 RUBY VERSION
-   ruby 2.5.8p224
+   ruby 2.6.6p146
 
 BUNDLED WITH
    2.1.4

app.rb

@@ -2,6 +2,7 @@
 
 require "bundler/setup"
 require 'kafka'
+require "concurrent/map"
 require "sinatra"
 require 'tempfile'