title: Rake doesn't know how to build task 'db:migrate'
body: <p>I've created an app using Ruby on Rails and Spree.</p>
  
  <p>After changing my SQlite database to PostgreSQL I needed to migrate my database. 
  But when I try to run rake db:migrate, rake db:migrate RAILS_ENV=development, bin/rake db:migrate RAILS_ENV=development I get the same error.</p>
  
  <pre><code>rake aborted!
  Don't know how to build task 'db:migrate:up' (see --tasks)
  /usr/local/rvm/gems/ruby-2.3.0/gems/rake-12.3.0/exe/rake:27:in `&lt;top (required)&gt;'
  /usr/local/rvm/gems/ruby-2.3.0/bin/ruby_executable_hooks:15:in `eval'
  /usr/local/rvm/gems/ruby-2.3.0/bin/ruby_executable_hooks:15:in `&lt;main&gt;'
  (See full trace by running task with --trace)
  </code></pre>
  
  <p><strong>When I run --trace I get the following response</strong></p>
  
  <pre><code>** Invoke default (first_time)
  ** Invoke spec (first_time)
  ** Execute spec
  /usr/local/rvm/rubies/ruby-2.3.0/bin/ruby -I/usr/local/rvm/gems/ruby-2.3.0/gems/rspec-core-3.7.0/lib:/usr/local/rvm/gems/ruby-2.3.0/gems/rspec-support-3.7.0/lib /usr/local/rvm/gems/ruby-2.3.0/gems/rspec-core-3.7.0/exe/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb
  Could not load dummy application. Please ensure you have run `bundle exec rake test_app`
  ** Execute default
  </code></pre>
  
  <p>Does any of you know what the cause of the problem is and what I should do?</p>
  
  <p><strong>Rakefile</strong></p>
  
  <pre><code>require 'rubygems'
  require 'rake'
  require 'rake/testtask'
  require 'rspec/core/rake_task'
  require 'spree/testing_support/common_rake'
  
  
  RSpec::Core::RakeTask.new
  
  task default: :spec
  
  desc "Generates a dummy app for testing"
  task :test_app do
    ENV['LIB_NAME'] = 'spree/frontend'
    Rake::Task['common:test_app'].invoke
  end
  </code></pre>