Update Travis configuration to use new alias

Travis CI deprecated and remove the rbx-2.0 alias after rbx project
merged stuff back to master branch.

.travis.yml

@@ -2,7 +2,7 @@ rvm:
   - 1.8.7
   - 1.8.7
   - 1.9.2
   - 1.9.2
   - ree
   - ree
-  - rbx-2.0
+  - rbx-18mode
 
 
 before_script: "sudo ntpdate -ub ntp.ubuntu.com pool.ntp.org; true"
 before_script: "sudo ntpdate -ub ntp.ubuntu.com pool.ntp.org; true"
 script: "bundle exec rake clean test cucumber"
 script: "bundle exec rake clean test cucumber"