I got this:

$ gem install passenger

Fetching: passenger-4.0.5.gem (100%)

ERROR:  While executing gem … (Gem::FilePermissionError)

You don’t have write permissions for the /Users/snowcrash/.rvm/gems/ruby-2.0.0-p195 directory.

Fixed like this:

rvm get head

rvm fix-permissions

See also:

http://stackoverflow.com/questions/17248872/error-while-executing-gem-gemfilepermissionerror