On your terminal, try running:
which -a ruby
This will output all the installed Ruby versions (via RVM, or otherwise) on your system in your PATH. If 1.8.7 is your system Ruby version, you can uninstall the system Ruby using:
sudo apt-get purge ruby
Once you have made sure you have Ruby installed via RVM alone, in your login shell you can type:
rvm --default use 2.0.0
You don’t need to do this if you have only one Ruby version installed.
If you still face issues with any system Ruby files, try running:
dpkg-query -l '*ruby*'
This will output a bunch of Ruby-related files and packages which are, or were, installed on your system at the system level. Check the status of each to find if any of them is native and is causing issues.