Fix #27, workspaces don’t list configurations

This check isn’t needed because `xcodebuild` will work even if an 
unknown configuration is passed in by using the default configuration
Merge branch 'master' into remove-configuration-check

Conflicts:
	lib/shenzhen/commands/build.rb
Fix #27: Treat configuration only as an override.

remove determine_configuration!
remove @configuration reference from build description. May not exist
Merge remote-tracking branch 'codecaffine/remove-configuration-check'

Conflicts:
	lib/shenzhen/commands/build.rb