Middleman: command not found

Middleman used to work and now it doesn’t. I know nothing about Ruby and would like to avoid the need to become an expert in it. I would prefer not to upgrade Middleman, if at all possible. I don’t use a Ruby manager and I hope I don’t need to.

$ bundle exec middleman build
bundler: command not found: middleman

$ bundle list middleman
/Library/Ruby/Gems/2.0.0/gems/middleman-3.4.0

$ gem env
RubyGems Environment:
  - RUBYGEMS VERSION: 2.0.14.1
  - RUBY VERSION: 2.0.0 (2015-12-16 patchlevel 648) [universal.x86_64-darwin16]
  - INSTALLATION DIRECTORY: /Library/Ruby/Gems/2.0.0
  - RUBY EXECUTABLE: /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby
  - EXECUTABLE DIRECTORY: /usr/local/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - universal-darwin-16
  - GEM PATHS:
     - /Library/Ruby/Gems/2.0.0
     - /Users/alan/.gem/ruby/2.0.0
     - /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/gems/2.0.0
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => false
     - :backtrace => false
     - :bulk_threshold => 1000
     - "gem" => "-N -n"
  - REMOTE SOURCES:
     - https://rubygems.org/

$ which ruby
/usr/bin/ruby

$ which gem
/usr/bin/gem

$ which bundle
/usr/local/bin/bundle

$ echo $PATH
/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/local/ant/bin:/usr/local/mysql/bin:/Library/Java/JavaVirtualMachines/1.8.0.jdk/Contents/Home/bin:/Users/alan/bin:/SharedApplications/android-sdk-macosx/tools:/Library/Ruby/Gems/2.0.0

$ bundle doctor
The Gemfile's dependencies are satisfied
No issues found with the installed bundle

Problem solved:

https://github.com/bundler/bundler/issues/5996