Specify -c .rubocop.yml for bin/rubocop

This avoids the problem where an existing .rubocop.yml is found
in the user's home directory that causes errors or misconfiguration.

It also speeds up the run by about 13%, presumably because it doesn't
need to look for the right config file.

bin/rubocop

@@ -6,4 +6,6 @@ $LOAD_PATH.unshift(File.expand_path("../bundler/lib", __dir__))
 ENV["BUNDLE_GEMFILE"] = File.expand_path("../tool/bundler/lint_gems.rb", __dir__)
 require "bundler/setup"
 
+ARGV.unshift("--config", ".rubocop.yml")
+
 load Gem.bin_path("rubocop", "rubocop")