When unpacking the prawn gem, it would be nice to be able to run rake manual to generate the manual. However, the manual/ folder is not included with the gem, and rake also expects a Gemfile to be present.
Add Gemfile and manual/ to gem manifest
Allows manual to be built from an unpacked gem. Closes #305.
This has been fixed on the master branch and will go out in the next gem release of Prawn. Thanks!