Add ZDI as reference authority

MSP-9325

Add ZDI as an authority for site reference in addition to the changes to
Mdm::Authority for the cache in prepartion for the merge of master to
feature/module-cache-construction, which will include usage of the ZDI
authority in module info.
Use Metasploit::Model::Authorities for Msf::SiteReference

MSP-9325

Remove case statement for converting ctx_id and ctx_val to a URL and use
the Metasploit::Model::Authorities in a similar manner to how they are
used for Mdm::Reference.
Use updated :metasploit_model_reference_zdi_designation

MSP-9325
bundle exec rake commands for travis-ci

MSP-9228

Fix "You have already activated rake 10.1.1, but your Gemfile requires
rake 10.1.0."

(cherry picked from commit 75f56f1)
Don't compare transformed instance to nil

MSP-9325

For transformed classes that override ==, using `== nil` and `!= nil`
can return incorrect results, so use the more idiomatic and correct
`nil?`, which is only true for `nil`.
Merge branch 'feature/zdi-authority' into feature/module-cache-construction
Closes #21