function update_bundle_path() {
  if [[ -f .bundle/config ]]; then
    if [[ ! -f .bundle/path || .bundle/path -ot .bundle/config ]]; then
      ruby -rbundler -e 'puts Bundler.bundle_path' > .bundle/path
    fi
  fi
}

push chpwd_functions update_bundle_path

function accept-line-with-bundler() {
  if [[ -r .bundle/path ]]; then
    set -- ${=BUFFER}
    if [[ -x $(< .bundle/path)/bin/$1 ]]; then
      BUFFER="bundle exec $BUFFER"
    fi
  fi
  zle accept-line
}

bindkey '^M' accept-line-with-bundler