Using Terminal or the command line to install anything is just great when things go smoothly however more often than not a certain dependency is missing or a recent update elsewhere has conflicted with whatever it is you were running perfectly fine before.
And so to the issue
Upon trying to install/reinstall Jekyll I got this error message:
The recent increased security changes in OS X El Capitan (called System Integrity Protection) now mean that in order to install Jekyll we need to explicitly specify a directory which isn’t protected by this. The default location for installation is the Ruby > Gems directory however it’s protected now. Therefore we specify a local directory:
Likewise the same command can be used for installing other Ruby Gems where this problem occurs.