CNB needs Bundler 2

gistfile1.txt

$ pack build --builder heroku/buildpacks sesc:master
Defaulting app directory to current working directory /Users/ssilver/work/sesc (use --path to override)
Using user-provided builder image heroku/buildpacks
Pulling builder image heroku/buildpacks (use --no-pull flag to skip this step)
latest: Pulling from heroku/buildpacks
Digest: sha256:9f86541d462ca1d812028104566c6a47e37445a27ad55f729b0182a6614c1085
Status: Image is up to date for heroku/buildpacks:latest
Selected run image heroku/pack:18 from builder heroku/buildpacks
Pulling run image heroku/pack:18 (use --no-pull flag to skip this step)
18: Pulling from heroku/pack
Digest: sha256:2230feb0c968f556fc2bca711595fda6355cd2a8c19ecb4d5b483abc30e51c3a
Status: Image is up to date for heroku/pack:18
Using cache image pack-cache-aa30cbc53859
===> DETECTING
[detector] Trying group of 2...
[detector] ======== Output: Ruby ========
[detector] Ruby
[detector] ======== Results ========
[detector] Ruby: pass
[detector] Procfile: pass
===> RESTORING
[restorer] cache image 'pack-cache-aa30cbc53859' not found, nothing to restore
===> ANALYZING
[analyzer] WARNING: image 'sesc:master' not found or requires authentication to access
[analyzer] WARNING: image 'sesc:master' has incompatible 'io.buildpacks.lifecycle.metadata' label
===> BUILDING
[builder] 
[builder]  !
[builder]  !     You must use Bundler 2 or greater with this lockfile.
[builder]  !
[builder] /tmp/d20190509-17-yr7bnd/bundler-1.15.2/gems/bundler-1.15.2/lib/bundler/lockfile_parser.rb:108:in `warn_for_outdated_bundler_version': You must use Bundler 2 or greater with this lockfile. (Bundler::LockfileError)
[builder] 	from /tmp/d20190509-17-yr7bnd/bundler-1.15.2/gems/bundler-1.15.2/lib/bundler/lockfile_parser.rb:95:in `initialize'
[builder] 	from /buildpacks/heroku_ruby/0.0.1/lib/language_pack/helpers/bundler_wrapper.rb:130:in `new'
[builder] 	from /buildpacks/heroku_ruby/0.0.1/lib/language_pack/helpers/bundler_wrapper.rb:130:in `block in parse_gemfile_lock'