Remove deprecated has_rdoc= from gemspec
Remove deprecated has_rdoc= from gemspec

Got this warning:

> 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 /home/henrik/auctionet/tmp/devbox/gems/bundler/gems/elk-5e762f7b2ad2/elk.gemspec:20.
jage/elk