Update Gemfile

It complained about:
```
root@pa-vagrant:~/railsconf2013# BUNDLE_GEMFILE=testapp/Gemfile bundle exec rake db:create --rakefile testapp/Rakefile
rake aborted!
Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes.
/root/railsconf2013/testapp/config/application.rb:7:in `<top (required)>'
/root/railsconf2013/testapp/Rakefile:4:in `require'
/root/railsconf2013/testapp/Rakefile:4:in `<top (required)>'
(See full trace by running task with --trace)
```
Uncommenting therubyracer fixed it :)