Enable fully versioned binary archive creation

Set environment variable FULL_VERSION to enable creation of fully
versioned 2.1.x binary archives including patch level.

Starting with Ruby 2.1, release version follow a semantic versioning
scheme. Setting envar FULL_VERSION allows one to create binary
archives like `ruby-2.1.2-p117-i386-mingw32.7z` by invoking
`ruby21:package:archive`.