I get an error when I used that ...
The source must be an absolute URI. For example: (ArgumentError) source 'https://rubygems.org'
Ended up with
ruby -rbundler/inline -e "gemfile(true) do; source 'https://rubygems.org/'; gem 'rails', '5.1.0.beta1'; end"
There seems to be a a rubygems issue with installing pre release versions of gems