warning: constant ::Fixnum is deprecated When generating new model
This warnings appear because you are using ruby 2.4.0.
This version introduced this change: Unify Fixnum and Bignum into Integer
See here for the announcement: https://www.ruby-lang.org/en/news/2016/12/25/ruby-2-4-0-released/
The warnings come from the activesupport gem which is part of rails and will be fixed in an upcoming release.
For now you can just ignore those warnings.
Update: Rails 5.0.2 has been released, which gets rid of the warnings.
I fixed mine by updating rails
bundle update rails