The ruby version 1.8.7 seems to be your system ruby.
Normally you can choose the ruby version you'd like, if you are using rvm with following. Simple change into your directory in a new terminal and type in:
rvm use 2.0.0
You can find more details about rvm here: http://rvm.io Open the website and scroll down, you will see a few helpful links. "Setting up default rubies" for example could help you.
Update: To set the ruby as default:
rvm use 2.0.0 --default