has_rdoc= is deprecated with no replacement

[efranco89]

NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It will be removed on or after 2018-12-01.
Gem::Specification#has_rdoc= called from /zipruby-compatibility-with-rubyzip-fork-f0e0668599cd/zipruby/zipruby-compat.gemspec:10.