Upgrade rspec to >= 3.9.0

[dblock]

Versions of rspec >= 3.9.0 fail tests unexpectedly, e.g. https://github.com/ruby-grape/grape/runs/5347064188?check_suite_focus=true

  1) Grape::DSL::Routing.version sets a version for route
     Failure/Error: namespace_inheritable(:version_options, options)

       #<#<Class:0x000055f796d81f10> (class)> received :namespace_inheritable with unexpected arguments
         expected: (:version_options, {:using=>:path})
              got: (:version_options, {:using=>:path})
     # ./lib/grape/dsl/routing.rb:48:in `version'
     # ./spec/grape/dsl/routing_spec.rb:23:in `block (3 levels) in <module:DSL>'

This is obviously wrong since the parameters received are identical to the expectation.

Possibly a bug in rspec-expectations.

In #2246 I locked the version of rspec to 3.9.0.