This repo has some handy handy hooks to run
npm install whenever you:
git checkouta new branch with a different Gemfile or package.json
git pulla change to Gemfile or package.json
git clone https://gist.github.com/5869846.git hooks && cd hooks && chmod +x install
./install [path to folder where you keep all your repos]
This will install these hooks globally, and then re-init any repos it finds, installing your shiny new hooks along the way.
You can uninstall these hooks by running
./install [path to your folders] remove.