Add support for overriding publish platform/version

This is useful in situations where packages that are build on one
platform/version are tested and released on 1..n other platforms/versions.

For example, one might build on Ubuntu 10.04 and test/publish on Ubuntu
11.04, 12.04 and Debian 7.
Expose platform/version override options on `omnibus publish` CLI
Add features for new CLI flags in publish subcommand
Merge pull request #272 from opscode/schisamo/publisher-override-release-platform

Add support for overriding publish platform/version
Configure Travis CI to monitor stable branches
Remove deprecated Omnibus.config method
Remove deprecated Omnibus.project_root
Remove deprecated Omnibus.root
Remove deprecated BuildVersion.full
Remove deprecated Builder.project_root
Remove deprecated Config.fetch ([])
Remove deprecated Config.install_path_cache_dir
Remove deprecated Config.override_file
Remove deprecated Config.software_gem
Remove deprecated Project.install_path
Remove deprecated Project.platform_version
Remove deprecated Project.platform
Remove deprecated Project.platform_family
Remove deprecated Project.machine
Remove deprecated Project.config