Deny installation into later than .pkg target version

- set attributes "min" and "before" of os-version in distribution xml.
- remove "os" key from product.plist.

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2439 f4f36d40-9719-0410-9868-dde021a23eda