~> 1.4.0
means >= 1.4.0 and < 1.5.0
Since there is not > 1.4.0 just install the 1.4.0 version with
sudo gem install multi_json --version 1.4.0
Notice that this can crash some other gems. Installing gems system wide is bad, but I guess you are not developing in ruby right? So you should be ok with this