Refactor package ruby-nanoc

Do not ship a license file with the package. Instead, install the license file
provided by the gem itself. Add `makedepends` and `options` arrays. Add a
comment explaining that one of namcap's warnings is spurious. Replace `sha1sums`
and `md5sums` checksum arrays with `sha256sums` checksum array. Reorder
variables, and remove some others entirely. Remove some quotation marks, convert
some double quotes into single quotes...