If your project uses another version than your default Ruby, RubyMine will give you incorrect inspections, for example.\
Here is how to switch which Ruby you use in RubyMine.
Ctrl+Alt+S
)Though it may seem you are changing a global setting here, this is in fact a per-project setting, as are all things you change in the "Project Settings [your_project_name]" area of the global settings dialog.
When you switch to another project, RubyMine will give you inspections and alike using that project's Ruby settings.