Remove ENV['TRAVIS']

Rather than replace `ENV['TRAVIS']` with `ENV['CI']`, I think we can
remove this condition because `ENV['TRAVIS']` is not valid at Buildkite
then Rails CI has been running with `PROCESS_COUNT` about 9 month
since this e485c14a3e

actionpack/test/abstract_unit.rb

@@ -13,11 +13,7 @@
   Encoding.default_external = Encoding::UTF_8
 end
 
-if ENV["TRAVIS"]
-  PROCESS_COUNT = 0
-else
-  PROCESS_COUNT = (ENV["MT_CPU"] || 4).to_i
-end
+PROCESS_COUNT = (ENV["MT_CPU"] || 4).to_i
 
 require "active_support/testing/autorun"
 require "abstract_controller"