Rails uninitialized constant Bundler (NameError)

These commands from the bundler FAQ did the trick:

#remove project-specific settings
rm -rf .bundle/

# remove project-specific cached gems and repos
rm -rf vendor/cache/

# remove the saved resolve of the Gemfile
rm -rf Gemfile.lock

and then rebundling with bundle install

edit: heroku deploy was also failing to start up because it starts the server with bin/rails s and my binstubs were all wonky. to fix this: BUNDLE INSTALL --BINSTUBS, again from bundler docs

I just removed the lock using rm -rf Gemfile.lock within my project directory's root folder, and re-ran bundle install and everything works perfect now.