rbenv: no such command `install’というエラーが出力された時．

# 結局
$ brew install ruby-build

# 以下，詳細
# 失敗
$ rbenv install --list
rbenv: no such command `install'

# GitHubを確認
sstephenson/rbenv
# "$ brew install rbenv ruby-build"とある．
# '/usr/local/Cellar/rbenv'はあるが, '/usr/local/Cellar/ruby-build'は無い．

# ruby-buildが必要
$ brew install ruby-build

# 成功
$ rbenv install --list
Available versions:
1.8.6-p383
1.8.6-p420
(以下省略)