Those files are lost once the build is finished. If you want to read them, you should add a cat
command to print out to the log you see.
before_script: cat /home/travis/.rvm/log/*_rbx-2.2.4/rubygems.install.log
If the install command is failing, then you should override install
to install the gem for which the installation is failing:
install: gem install XXX || cat /home/travis/.rvm/log/*_rbx-2.2.4/rubygems.install.log