Move Puppet version into lib/puppet/version.rb

Without this patch the entire Puppet library must be loaded in order to
get the version. This is problematic for executed metadata files like
the rubygems gemspec or bundler Gemfile.

This patch fixes the problem by moving the Puppet.version method and
Puppet::VERSION constant into an isolated small file. Gemfile and
gemspec files may determine the version using require 'puppet/version'