Run RuboCop on spec/truffle specs

* Using the ruby/spec configuration, as .rubocop.yml doesn't work well
  for specs.

Author: eregon

tool/jt.rb

@@ -1961,7 +1961,9 @@ def rubocop(*args)
       "GEM_PATH" => gem_home,
       "PATH" => "#{gem_home}/bin:#{ENV['PATH']}"
     }
-    if args.empty? or args.all? { |arg| arg.start_with?('-') }
+    if i = args.index('--specs')
+      args[i..i] = %w[-c spec/ruby/.rubocop.yml spec/truffle]
+    elsif args.empty? or args.all? { |arg| arg.start_with?('-') }
       args += RUBOCOP_INCLUDE_LIST
     end
     sh env, "ruby", "#{gem_home}/bin/rubocop", *args
@@ -2031,6 +2033,7 @@ def lint(*args)
     check_dsl_usage unless args.delete '--no-build'
     check_filename_length
     rubocop
+    rubocop('--specs')
     sh "tool/lint.sh"
     checkstyle
     check_parser