Cannot start a new application.

I installed RoR on my MacOS X using this guide http://ryanbigg.com/2010/12/ubuntu-ruby-rvm-rails-and-you/

Here are my versions:

$ ruby --version
ruby 1.8.7 (2010-01-10 patchlevel 249) [universal-darwin11.0]
$ rails --version
Rails 3.2.3
$ gem --version
1.8.23

I was unable to start the application, so I searched and modified the Gemfile using this tutorial: http://ruby.railstutorial.org/ruby-on-rails-tutorial-book

Now this is my gemfile:

source 'https://rubygems.org'

gem 'rails', '3.2.3'

# Bundle edge Rails instead:
# gem 'rails', :git => 'git://github.com/rails/rails.git'

group :development do
  gem 'sqlite3', '1.3.5'
end

# Gems used only for assets and not required
# in production environments by default.
group :assets do
  gem 'sass-rails',   '3.2.4'
  gem 'coffee-rails', '3.2.2'

  # See https://github.com/sstephenson/execjs#readme for more supported runtimes
  # gem 'therubyracer', :platform => :ruby

  gem 'uglifier', '1.2.3'
end

gem 'jquery-rails', '2.0.0'

# To use ActiveModel has_secure_password
# gem 'bcrypt-ruby', '~> 3.0.0'

# To use Jbuilder templates for JSON
# gem 'jbuilder'

# Use unicorn as the app server
# gem 'unicorn'

# Deploy with Capistrano
# gem 'capistrano'

# To use debugger
# gem 'ruby-debug19', :require => 'ruby-debug'

When I launch the application using

rails server

I got it:

=> Booting WEBrick
=> Rails 3.2.3 application starting in development on http://0.0.0.0:3000
=> Call with -d to detach
=> Ctrl-C to shutdown server
Exiting
/Library/Ruby/Gems/1.8/gems/activesupport-3.2.3/lib/active_support/dependencies.rb:245:in `load': /Users/giacomo/websrc/rubyrails-app/config/initializers/session_store.rb:3: syntax error, unexpected ':', expecting $end (SyntaxError)
...sion_store :cookie_store, key: '_rubyrails-app_session'
                              ^
    from /Library/Ruby/Gems/1.8/gems/activesupport-3.2.3/lib/active_support/dependencies.rb:245:in `load'
    from /Library/Ruby/Gems/1.8/gems/activesupport-3.2.3/lib/active_support/dependencies.rb:236:in `load_dependency'
    from /Library/Ruby/Gems/1.8/gems/activesupport-3.2.3/lib/active_support/dependencies.rb:245:in `load'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/engine.rb:588
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/engine.rb:587:in `each'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/engine.rb:587
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/initializable.rb:30:in `instance_exec'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/initializable.rb:30:in `run'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/initializable.rb:55:in `run_initializers'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/initializable.rb:54:in `each'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/initializable.rb:54:in `run_initializers'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/application.rb:136:in `initialize!'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/railtie/configurable.rb:30:in `send'
    from /Library/Ruby/Gems/1.8/gems/railties-3.2.3/lib/rails/railtie/configurable.rb:30:in `method_missing'
    from /Users/giacomo/websrc/rubyrails-app/config/environment.rb:5
    from /Users/giacomo/websrc/rubyrails-app/config.ru:4:in `require'
    from /Users/giacomo/websrc/rubyrails-app/config.ru:4
    from /Library/Ruby/Gems/1.8/gems/rack-1.4.1/lib/rack/builder.rb:51:in `instance_eval'
    from /Library/Ruby/Gems/1.8/gems/rack-1.4.1/lib/rack/builder.rb:51:in `initialize'
    from /Users/giacomo/websrc/rubyrails-app/config.ru:1:in `new'
    from /Users/giacomo/websrc/rubyrails-app/config.ru:1

How can i solve the problem?