rails - `autodetect': Could not find a JavaScript runtime.

I was following this guide:
https://devcenter.heroku.com/articles/getting-started-with-rails4


To get rails and heroku working.

But when I got to:

$ rails generate controller welcome

I got an error:

/home/tims/.rvm/gems/ruby-2.1.0/gems/execjs-2.0.2/lib/execjs/runtimes.rb:51:in `autodetect': Could not find a 
JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes. 
(ExecJS::RuntimeUnavailable)
        from /home/tims/.rvm/gems/ruby-2.1.0/gems/execjs-2.0.2/lib/execjs.rb:5:in `'
        from /home/tims/.rvm/gems/ruby-2.1.0/gems/execjs-2.0.2/lib/execjs.rb:4:in `'
        from /home/tims/.rvm/gems/ruby-2.1.0/gems/uglifier-2.4.0/lib/uglifier.rb:3:in `require'
        from /home/tims/.rvm/gems/ruby-2.1.0/gems/uglifier-2.4.0/lib/uglifier.rb:3:in `'
        from /home/tims/.rvm/gems/ruby-2.1.0@global/gems/bundler-1.5.3/lib/bundler/runtime.rb:76:in `require'
        from /home/tims/.rvm/gems/ruby-2.1.0@global/gems/bundler-1.5.3/lib/bundler/runtime.rb:76:in `block (2 levels) in require'
        from /home/tims/.rvm/gems/ruby-2.1.0@global/gems/bundler-1.5.3/lib/bundler/runtime.rb:72:in `each'
        from /home/tims/.rvm/gems/ruby-2.1.0@global/gems/bundler-1.5.3/lib/bundler/runtime.rb:72:in `block in require'
        from /home/tims/.rvm/gems/ruby-2.1.0@global/gems/bundler-1.5.3/lib/bundler/runtime.rb:61:in `each'
        from /home/tims/.rvm/gems/ruby-2.1.0@global/gems/bundler-1.5.3/lib/bundler/runtime.rb:61:in `require'
        from /home/tims/.rvm/gems/ruby-2.1.0@global/gems/bundler-1.5.3/lib/bundler.rb:131:in `require'
        from /home/tims/ruby/hsfeb/config/application.rb:7:in `'
        from /home/tims/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/commands.rb:43:in `require'
        from /home/tims/.rvm/gems/ruby-2.1.0/gems/railties-4.0.2/lib/rails/commands.rb:43:in `'
        from bin/rails:4:in `require'
        from bin/rails:4:in `
'

I fixed this by installing therubyracer gem:

gem install therubyracer

And then by uncommenting the therubyracer line in the project's Gemfile.

(Thanks to http://guides.rubyonrails.org/getting_started.html -
"Rails adds the therubyracer gem to Gemfile in a commented line for new apps and you can uncomment if you need it.")

---

Part 2

After starting the server and visiting / I got:

Started GET "/" for 192.168.0.20 at 2014-02-12 23:50:54 +1000

PG::ConnectionBad (FATAL:  Peer authentication failed for user "hsfeb"
):
  activerecord (4.0.2) lib/active_record/connection_adapters/postgresql_adapter.rb:831:in `initialize'
  activerecord (4.0.2) lib/active_record/connection_adapters/postgresql_adapter.rb:831:in `new'
  activerecord (4.0.2) lib/active_record/connection_adapters/postgresql_adapter.rb:831:in `connect'

This was easily fixed by following the instructions for setting up postgres, later in the document.

---

I'm added

ruby "2.1.0"

to my Gemfile instead of 2.0.0.