Useful rhc Commands
The command rhc tail -a appname will tail all application log files in the OPENSHIFT_LOG_DIR directory for the primary web gear of the application.
$ rhc tail -a myapp [--namespace mynamespace]
Errors and Solutions
You have already activated rack 1.5.2, but your Gemfile requires rack 1.6.0.
Using bundle exec may solve this. (Gem::LoadError)
ssh into the OpenShift app and in the app-root folder run
gem install rack