gemを入れてbundle install を行ったところ、以下のエラーが出ました。
Your bundle is locked to mimemagic (0.3.5), but that version could not be found
in any of the sources listed in your Gemfile. If you haven't changed sources,
that means the author of mimemagic (0.3.5) has removed it. You'll need to update
your bundle to a version other than mimemagic (0.3.5) that hasn't been removed
in order to install.
んん、よく分からん。。とりあえずGemfile.lockで関係してそうな内容を消して再度bundle install をしたら直りました。