InstallError installing ember-cli-rails

I'm new to Ruby so I'm sure there's something I've overlooked, but I don't know what.

I'm trying to install a Rails app in a bundle.

$ bundle install --path vendor/bundle

Many gems work, but

Installing ember-cli-rails 0.7.0
Gem::InstallError: ember-cli-rails requires Ruby version >= 2.1.0.

I think I'm running an acceptable version of Ruby...

$ bundle exec ruby --version
ruby 2.2.3p173 (2015-08-18 revision 51636) [x86_64-darwin14]   
$ ruby --version
ruby 2.2.3p173 (2015-08-18 revision 51636) [x86_64-darwin14]

Any ideas of what's wrong?