Add Appledoc 2.1 to continue to support projects with undocumented
enumerated and bitmask types.

This is identical to the last version of homebrew/appledoc that referenced 2.1
(mxcl/homebrew@33aa810) except for the comments at top and the replacement
of `def test` with a block-style test (noting that that modification was made to
the last version of Appledoc in homebrew-versions (ec73b7bbf3)).