Here's what I did to get things working.

Self explanatory.

Hit it: https://developer.apple.com/downloads/index.action?name=for%20Xcode%20-#

Make sure you get the package for 10.10.

Copy/pasted from the site:

ruby -e "$(curl -fsSL https://raw.github.com/Homebrew/homebrew/go/install)"

Open up /usr/local/bin/brew with your favorite editor.

I had to comment out the brew system detection, so that the last 6-7 lines looked like this:

#BREW_SYSTEM=$(uname -s | tr "[:upper:]" "[:lower:]")
#if [ "$BREW_SYSTEM" = "darwin" ]
#then
#    exec "$BREW_LIBRARY_DIRECTORY/brew.rb" "$@"
#else
    exec ruby -W0 "$BREW_LIBRARY_DIRECTORY/brew.rb" "$@"
#fi

It looks like Homebrew somehow depends on Ruby 1.8 despite even Mavericks shipping with version 2. So until Homebrew fixes this, we'll symlink our system version of Ruby to the one Homebrew is looking for (in Versions/1.8)

Create the folder:

sudo mkdir -p /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin

Then symlink the binary:

sudo ln -s /usr/bin/ruby /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby

My subsequent brew install commands seemed to work fine (ruby-build, redis, etc...). Along with rbenv, etc.

You'll probably need to install Bundler with gem install bundler if you haven't already.

Leave a comment if something didn't work.