Include rdoc as a dependency

We've had some reports (#1342, #1809) of `rdoc` not being installed
in some environments, causing Gollum to error in unpleasant ways.

This isn't an issue that's unique to Gollum, and I don't think `rdoc`
should actually be a hard dependency of this project.  But since we can
side-step the issue by requiring `rdoc`, something that most users who
install Gollum via RubyGems would already have installed, I don't think
this is a horrible solution.

gollum.gemspec

@@ -20,6 +20,7 @@ Gem::Specification.new do |s|
   s.rdoc_options = ['--charset=UTF-8']
   s.extra_rdoc_files = %w[README.md LICENSE]
 
+  s.add_dependency 'rdoc', '~> 6'
   s.add_dependency 'gollum-lib', '~> 5.1'
   s.add_dependency 'kramdown', '~> 2.3'
   s.add_dependency 'kramdown-parser-gfm', '~> 1.1.0'