What an incredible WWDC! You've probably been checking out the new bits and you've collected several versions of Xcode on your machine.
Traditionally, you'd switch active xcodebuild versions using the xcode-select --switch command-line utility. But this requires sudo and switches the active developer directory system-wide, both of which can be undesirable particularly for CI builds.
It turns out you can specify a per-build developer directory on the command line without xcode-select! The trick is twofold: assign a value to the DEVELOPER_DIR environment variable and run your build with xcrun.
I standardize my build commands with rake tasks. Here's a rake test task you might use, perhaps in your xcode-7 branch, to execute a single build using an alternate Xcode:
desc 'Run all application tests.'
task :test do
dev_dir = ENV['DEVELOPER_DIR'] || ""
# Path to desired Xcode beta
ENV['DEVELOPER_DIR'] = '/Applications/Xcode-beta.app/Contents/Developer'
sh "set -o pipefail && xcrun -sdk iphonesimulator "+
"xcodebuild ... etc ... "+
"clean test | bundle exec xcpretty -ct"
ENV['DEVELOPER_DIR'] = dev_dir
end