Heroku ruby buildpack error - undefined local variable or method `install_language_pack_gems'

Question

I just upgraded my Rails 3.2.16 app's ruby version from 1.9.2 to 2.0.0 and everything is working fine locally. However, when trying to push to Heroku, it's failing to compile.

It detects the ruby version, starts installing dependencies, then crashes:

-----> Compiling Ruby/Rails
-----> Using Ruby version: ruby-2.0.0
-----> Installing dependencies using 1.5.2
   Ruby version change detected. Clearing bundler cache.
   Old: ruby 1.9.3p484 (2013-11-22 revision 43786) [x86_64-linux]
   New: ruby 2.0.0p353 (2013-11-22 revision 43784) [x86_64-linux]
sh: Syntax error: EOF in backquote substitution
sh: Syntax error: EOF in backquote substitution
/app/tmp/buildpacks/ruby/lib/language_pack/ruby.rb:760:in `block in purge_bundler_cache': undefined local variable or method `install_language_pack_gems' for #<LanguagePack::Rails3:0x00000000c1d090> (NameError)

Seems like something simple, but I don't particularly want to mess with the buildpack.

This also happens for an app running on Ruby 2.0

-----> Installing dependencies using 1.5.2
       Ruby version change detected. Clearing bundler cache.
       Old: ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-linux]
       New: ruby 2.0.0p353 (2013-11-22 revision 43784) [x86_64-linux]
sh: Syntax error: EOF in backquote substitution
sh: Syntax error: EOF in backquote substitution
/app/tmp/buildpacks/ruby/lib/language_pack/ruby.rb:760:in `block in purge_bundler_cache': undefined local variable or method `install_language_pack_gems' for #<LanguagePack::Rails4:0x00000002084a60> (NameError)

Answer 1

Seems like it is not using the latest ruby buildpack. I was able to solve this by setting the BUILDPACK_URL

$ heroku config:set BUILDPACK_URL=https://github.com/heroku/heroku-buildpack-ruby