Heroku Rails app immediately crashes on HTTP request
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Ruby 1.9.3 and Rails 3.2 on Heroku
Heroku’s newest stack, “Celadon Cedar,” supports Rails 3.2 but installs Ruby 1.9.2 by default. Ruby 1.9.3 is recommended for Rails 3.2.
You can configure the Heroku environment to use Ruby 1.9.3.
Note: Heroku makes it clear that Ruby 1.9.3 on Heroku is experimental, which means “no support, the ruby_version will change in the future, and this feature may change or be removed without warning.” In response to an inquiry on January 31, 2012, Heroku said, “there is no timeline yet” to fully support Ruby 1.9.3.
Note: Installing Ruby 1.9.3 on Heroku previously (before March 19, 2012) required installation of the heroku-labs plugin. The heroku-labs plugin is deprecated and its functionality is now incorporated in the heroku client gem.
Taken from here