/usr/local can no longer be chown'd in High Sierra

As a workaround, execute the command to the subdirectory.

The error message was:
```
chown: changing ownership of '/usr/local': Operation not permitted
```

See also:
- Homebrew/brew#3228
- https://stackoverflow.com/questions/46459152/cant-chown-usr-local-for-homebrew-in-osx-10-13-high-sierra

bin/brewfile.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 set -eu
-sudo chown -R $(whoami):admin $(brew --prefix)
+sudo chown -R $(whoami):admin $(brew --prefix)/*
 cd $(brew --repository) && git checkout master
 cd -
 

bin/convert_brewfile_to_shell_script.rb

@@ -1,7 +1,7 @@
 File.open ARGV[0] do |brewfile|
   puts '#!/bin/sh'
   puts 'set -eu'
-  puts 'sudo chown -R $(whoami):admin $(brew --prefix)'
+  puts 'sudo chown -R $(whoami):admin $(brew --prefix)/*'
   puts 'cd $(brew --repository) && git checkout master'
   puts 'cd -'