brew doctorしたら、以下の様な出力が得られた。

$ brew doctor
Error: Failed to import: dotjs
No available formula for dotjs
Error: Failed to import: fbcmd
No available formula for fbcmd
Error: Failed to import: kindlegen
No available formula for kindlegen
Error: Failed to import: kzipmix
No available formula for kzipmix
Error: Failed to import: live-f1
No available formula for live-f1
Error: Failed to import: mandoc
No available formula for mandoc
Error: Failed to import: mongodb-dev
No available formula for mongodb-dev
Error: Failed to import: mono
No available formula for mono
Error: Failed to import: openmotif
No available formula for openmotif
Error: Failed to import: perlbrew
No available formula for perlbrew
Error: Failed to import: pngout
No available formula for pngout
Error: Failed to import: princexml
No available formula for princexml
Error: Failed to import: rtmpdump-yle
No available formula for rtmpdump-yle
Error: Failed to import: sc
No available formula for sc
Error: Failed to import: simian
No available formula for simian
Error: Failed to import: svm_light
No available formula for svm_light
Error: Failed to import: terminus-font
No available formula for terminus-font
Error: Failed to import: tmux-iterm2
No available formula for tmux-iterm2
Error: Failed to import: tsocks
No available formula for tsocks

よくわからんけど直したいのでぐぐったら、このページが見つかった
https://github.com/mxcl/homebrew/issues/18237
ここのコメントの通り

brew prune
brew cleanup
brew update
brew tap --repair
brew doctor

したら直った。