Just like the topic!
May I know what is going on?
You can look at it this way.
The result should be
This is ruby2.0 preinstalled by the system.
Brew install ruby will be installed in
ruby -v ruby 2.0.0p481 (2014-05-08 revision 45883) [universal.x86_64-darwin14] @caoh ➜ ~ /usr/local/Cellar/ruby/2.2.3/bin/ruby -v ruby 2.2.3p173 (2015-08-18 revision 51636) [x86_64-darwin14]