title: Rails 5 db migration: how to fix ActiveRecord::ConcurrentMigrationError
body: <p>A previous rake db:rollback stalled. Now when attempting a new migration we get the following error:</p>
  
  <pre><code>rake aborted!
  ActiveRecord::ConcurrentMigrationError: 
  
  Cannot run migrations because another migration process is currently running.
  
  /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-    5.1.4/lib/active_record/migration.rb:1315:in `with_advisory_lock'
  /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/migration.rb:1148:in `migrate'
  /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/migration.rb:1007:in `up'
  /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/migration.rb:985:in `migrate'
  /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:171:in `migrate'
  /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/railties/databases.rake:58:in `block (2 levels) in &lt;top (required)&gt;'
  /home/me/.rvm/gems/ruby-2.4.1/gems/rake-12.1.0/exe/rake:27:in `&lt;top (required)&gt;'
  /home/me/.rvm/gems/ruby-2.4.1/bin/ruby_executable_hooks:15:in `eval'
  /home/me/.rvm/gems/ruby-2.4.1/bin/ruby_executable_hooks:15:in `&lt;main&gt;'
  Tasks: TOP =&gt; db:migrate
  (See full trace by running task with --trace)
  </code></pre>
  
  <p>We are using Postresql</p>