olddoc 1.3.0

This release supports the newly-released rdoc 5.x, but maintains
support for rdoc 4.2+.  A new "source_code" accessor was
introduced to make us less git-centric to support other systems.

7 changes since 1.2.0:

      _head: fix Content-Type charset attribute
      add source_code accessor for tail footer
      build: dynamically generate version number
      _tail: do not mutate existing arrays
      gemspec: support rdoc 5.x
      build: avoid network usage in "install-gem" target
      doc: olddoc.5: document "source_code" field