@Mopr answer will possibly resolve, but did not work for me. I got a more complete answer from Enisto Pino Alcatraz issue 352 Which I am reposting here
1) Close Xcode
2) Remove previous defaults for Xcode 7.0
defaults delete com.apple.dt.Xcode DVTPlugInManagerNonApplePlugIns-Xcode-7.0
3) Uninstall Alcatraz
rm -rf ~/Library/Application\ Support/Developer/Shared/Xcode/Plug-ins/Alcatraz.xcplugin
4) Remove all packages installed via Alcatraz
rm -rf ~/Library/Application\ Support/Alcatraz/
5) Update the DVTPlugInCompatibilityUUID to the 7.1 in the plists of plugins:
find ~/Library/Application\ Support/Developer/Shared/Xcode/Plug-ins -name Info.plist -maxdepth 3 | xargs -I{} defaults write {} DVTPlugInCompatibilityUUIDs -array-add defaults read /Applications/Xcode\ 7.1.app/Contents/Info DVTPlugInCompatibilityUUID
6) Reset Xcode select
sudo xcode-select --reset
7) Open Xcode
8) Install Alcatraz
curl -fsSL https://raw.github.com/supermarin/Alcatraz/master/Scripts/install.sh | sh
9) Restart Xcode
10) Select "Load Bundles" in dialog box at Xcode start.