There are two ways to get over this annoying /usr/bin error
1. Install the same gem using RVM (Ruby Version Manager), but first, you'll need RVM. Here's how to install RVM on OSX
\curl -sSL https://get.rvm.io | bash -s stable
PS: If that didn't work, here is some extra help
2. "-n flag"
Install the same gem using the mysterious "-n flag"
sudo gem install cocoapods -n /usr/local/bin
3. "Create a ~/.gemrc file"
With the following content:
:gemdir: - ~/.gem/ruby install: -n /usr/local/bin
Now you can install using gem install without the -n flag.
sudo gem install git_swap