Remove deprecated has_rdoc from Gem::Specification

[skmessage]

A bundle update to the avanti-lms project when upgrading to use Ruby-2.4 produces the following warning:

Fetching https://github.com/peerlearning/mathjax-rails.git
Fetching https://github.com/peerlearning/materialize-sass.git
Fetching https://github.com/peerlearning/redis-namespace.git
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 /app/avanti-lms-ruby-upgrade-240/.local/ruby/2.4.0/bundler/gems/redis-namespace-941666c4f986/redis-namespace.gemspec:13.
...

The attached patch removes the deprecated 'has_rdoc' from Gem::Specification.