Rails gem installed but throws error

looks like i have a problem trying to install rails gem.

so i followed the steps from this tutorial, I am using Ubuntu 14.04, rbenv and ruby version 2.2.3

Basically my error is:

apparently I installed rails gem successfully

$ gem install rails -v 4.2.4

-> Successfully installed rails-4.2.4

but whenever i try to use rails command, for example:

$ rails -v

-> bin/rails:9:in require': cannot load such file -- rails/commands (LoadError) from bin/rails:9:in'

I searched everywhere and noone seem to have the same problem as I do. I dont really know what to do now, any help will be really appreciated.

Thanks in advance!!