Yosemite uses Ruby 2.0 where as my previously installed Homebrew was set to use 1.8. I did an upgrade from Mavericks to Yosemite just last week, but it was today that I set out using brew commands and realized, oh no! But I have a 2 minute fix that works just fine. 

Step 1

Change the first line of brew.rb (located in /usr/local/Library) to this:

#!/usr/bin/ruby -W0

Step 2

Follow these commands in order:

cd `brew --prefix`

git clean -f

git reset --hard FETCH_HEAD

git clean -fd

brew update

And you are done. Finally, you should see something like this if all went well.

Updated Homebrew from 3315455f to 3315455f.

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