環境はMac OSX Mavericks, Rails4, Ruby2.0
以下の２点を設定した
１．libeditをhomebrewで入れたreadlineに変更
http://qiita.com/irohiroki/items/c82657b5cb4bdb2aaac4
$ install_name_tool -change /usr/lib/libedit.3.dylib /usr/local/Cellar/readline/6.2.4/lib/libreadline.dylib /Users/shohei/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/x86_64-darwin12.4.0/readline.bundle
２．環境変数RUBYOPTに-Kuを設定（もしやこっちだけでもOK?）
http://stackoverflow.com/questions/5809412/global-settings-to-solve-invalid-multibyte-char-us-ascii-in-rails-3
.zshrcに追記
export RUBYOPT=-Ku
