Update platform_map with new build matrix label names

We recently updated the following matrix labels for the Jenkins build 
matrix:

build_os -> platform
machine_architecture -> architecture

OSC's release script relies on the values in this JSON file to map a 
directory to a certain platform, platform value and architecture.

pipeline test here:
http://andra.ci.opscode.us/job/chef-server-trigger-ad_hoc/5/downstreambuildview/

/cc @mmzyk