Stop testing with rbx, start testing with 1.9.3

.travis.yml

 rvm:
   - 1.8.7
   - 1.9.2
+  - 1.9.3
   - ree
-  - rbx-18mode
 
 before_script: "sudo ntpdate -ub ntp.ubuntu.com pool.ntp.org; true"
 script: "bundle exec rake clean test cucumber"