get rid of RSpec deprecation warning

Using `should_not` from rspec-expectations' old `:should` syntax without explicitly enabling the syntax is deprecated. Use the new `:expect` syntax or explicitly enable `:should` with `config.expect_with(:rspec) { |c| c.syntax = :should }` instead. Called from /Users/foxy/src/Retrospring/retrospring/spec/controllers/well_known/node_info_controller_spec.rb:84:in `block (4 levels) in <top (required)>'.

spec/controllers/well_known/node_info_controller_spec.rb

@@ -81,7 +81,7 @@ describe WellKnown::NodeInfoController do
       it "includes Twitter in outbound services" do
         subject
         parsed = JSON.parse(response.body)
-        parsed.dig("services", "outbound").should_not include("twitter")
+        expect(parsed.dig("services", "outbound")).to_not include("twitter")
       end
     end