In my particular instance I was trying to run bundle install on my first heroku app. I did all the items that @boobooninja listed (with slightly different paths, of course). What ended up working for me was a combination of two answers. I needed to
$ sudo su
$ env ARCHFLAGS="-arch x86_64" gem install pg -v '0.17.1'
By adding the -v 0.17.1 and specifying the version needed by heroku I was able to complete my bundle and proceed.
