Move the definition of the VERSION constant in lib/prawn/version.rb

Instead of encoding the version in a file text in the root of the gem,
encode the VERSION variable in a file under lib/prawn.

Having the files in lib/ depending on files in the root directory is not
practical for distributions wanting to package the library.

The VERSION file was also used in the gemspecs. Instead of reading it, the version number is directly loaded from the Prawn::VERSION constant. Is it considered ok to require files from lib/ in the gemspec?