engine_cart:prepare is idempotent when stanza is current

[dazza-codes]

$ bundle exec rake -T
rake build                                    # Build engine_cart-1.1.0.gem into the pkg directory
rake clean                                    # Remove any temporary products
rake clobber                                  # Remove any generated files
rake engine_cart:clean                        # Clean out the test rails app
rake engine_cart:console[rails_console_args]  # Start the internal test application using `rails console`
rake engine_cart:generate[fingerprint]        # Create the test rails app

## this PR add the "force" option
rake engine_cart:prepare[force]               # Prepare a gem for using engine_cart

rake engine_cart:regenerate                   # Regenerate the test rails app
rake engine_cart:server[rails_server_args]    # Start the internal test application using `rails server`
rake install                                  # Build and install engine_cart-1.1.0.gem into system gems
rake install:local                            # Build and install engine_cart-1.1.0.gem into system gems without network access
rake release[remote]                          # Create tag v1.1.0 and build and push engine_cart-1.1.0.gem to Rubygems
rake spec                                     # Run RSpec code examples

$ bundle exec rake engine_cart:prepare
      create  spec/test_app_templates
      create  spec/test_app_templates/lib/generators
      create  spec/test_app_templates/lib/generators/test_app_generator.rb
      append  .gitignore
      append  Gemfile
$ bundle exec rake engine_cart:prepare  # no-op, stanza is current
$ bundle exec rake engine_cart:prepare  # no-op, stanza is current

# force it to prepare again
$ bundle exec rake engine_cart:prepare[true]
       exist  spec/test_app_templates
       exist  spec/test_app_templates/lib/generators
   identical  spec/test_app_templates/lib/generators/test_app_generator.rb
      append  .gitignore
      append  Gemfile

[dazza-codes]

Really weird travis build failure and can't restart the build:

NameError: uninitialized constant EngineCart::Configuration::ERB

Works for me:

/users/dlweber/.rbenv/versions/2.4.1/bin/ruby -I/users/dlweber/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rspec-core-3.6.0/lib:/users/dlweber/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rspec-support-3.6.0/lib /users/dlweber/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/rspec-core-3.6.0/exe/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb
..../users/dlweber/.rbenv/versions/2.4.1/lib/ruby/2.4.0/fileutils.rb:120: warning: conflicting chdir during another chdir block
../users/dlweber/.rbenv/versions/2.4.1/lib/ruby/2.4.0/fileutils.rb:120: warning: conflicting chdir during another chdir block
/users/dlweber/.rbenv/versions/2.4.1/lib/ruby/2.4.0/fileutils.rb:120: warning: conflicting chdir during another chdir block
...

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