Require alpha arel

Without this change, bundler will resolve a gemfile which is pointing at
Rails master and not arel master. The error message that someone will
get from doing this will be incredibly unhelpful, as it'll be the result
of ActiveRecord being in a half-loaded that halted at `require
"arel/collectors/composite"`, which was subsequently rescued in
`"rails/all"`

activerecord/activerecord.gemspec

@@ -31,5 +31,5 @@
   s.add_dependency "activesupport", version
   s.add_dependency "activemodel",   version
 
-  s.add_dependency "arel", "~> 8.0"
+  s.add_dependency "arel", "9.0.0.alpha"
 end