Use latest Rails by default

Latest Rails includes ActiveCable tests, that are otherwise skipped when
testing compatibility of other RSpec projects, e.g. `rspec-expectation`
on CI.

Author: pirj

Gemfile

@@ -28,7 +28,7 @@ when /stable/
   MAJOR = 6
   MINOR = 0
 when nil, false, ""
-  MAJOR = 5
+  MAJOR = 6
   MINOR = 0
 else
   match = /(\d+)(\.|-)(\d+)/.match(RAILS_VERSION)