Improve readability of permissions code
Improve readability of permissions code

* Better reveal intent with `HOMEBREW_PREFIX` variable.
* Do less work by referencing `$USER` instead of `whoami`.

https://github.com/thoughtbot/laptop/issues/444

The $USER environment variable is not guaranteed to exist,
but $LOGNAME is defined by POSIX
(Environment Variables, section 8.3 Other Environment Variables).

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
http://unix.stackexchange.com/questions/76354/who-sets-user-and-username-environment-variables

iphilgood/laptop