I'm attempting to update software on my iMac with Capitan installed, using Homebrew. Every time I try I receive the error message that /usr/local must be writable. I am the admin user and have tried using sudo with no luck. Until recently I've not had this issue and was wondering if it's related to the latest OS X update and, how do I go about fixing it so this error message doesn't appear in Terminal? It's very frustrating as I now can't use HomeBrew. Thanks.