Fix: chef-solo command not found travis error.

.travis.yml

@@ -16,11 +16,11 @@ before_install:
   # According to http://docs.travis-ci.com/user/build-configuration/.
   # Update OS before installing prerequisites.
   - sudo apt-get update
-  # Install Chef Solo prerequisites.
-  - sudo apt-get install ruby ruby-dev libopenssl-ruby rdoc ri irb build-essential ssl-cert
 
 install:
   - sudo hostname localhost
+  # Set Ruby environment to use `gem`.
+  - rvm use 1.9.3
   # Install Chef Solo.
   # Chef Solo 11.4.4 is broken, so install a previous version.
   # The bug is planned to be fixed in 11.4.5 which haven't been released yet.