Bump dependencies

Update `activemodel`, `railties` and its dependencies.

It's related with security issues solved in version `4.0.2`:

https://groups.google.com/forum/#!topic/rubyonrails-security/9WiRn2nhfq0
https://groups.google.com/forum/#!topic/rubyonrails-security/niK4drpSHT4
https://groups.google.com/forum/#!topic/rubyonrails-security/pLrh6DUw998
https://groups.google.com/forum/#!topic/rubyonrails-security/5ZI1-H5OoIM