cask doctor: fix for non-standard installation location in ~
cask doctor: fix for non-standard installation location in ~

Symptom (relevant portions):
```
$ brew cask doctor
==> Homebrew-Cask Version
Homebrew-Cask 1.6.7-56-g9ebcef7
Homebrew/homebrew-cask (git revision fc24e; last commit 2018-06-09)
==> macOS
10.13.5
==> SIP
Enabled
...
==> Homebrew-Cask Install Location
<NONE>
==> Homebrew-Cask Staging Location
~/homebrew/Caskroom
...
==> Environment Variables
HOMEBREW_CASK_OPTS="--appdir=~/Applications"
LC_ALL="en_US.UTF-8"
PATH="/usr/bin:/bin:/usr/sbin:/sbin:~/homebrew/Library/Homebrew/shims/scm"
SHELL="/bin/bash"
Cask's Doctor Checkup: failed
 - The staging path ~/homebrew/Caskroom does not exist.
Error: There are some problems with your setup.
```

Analysis:

* The source code is
  [this](https://github.com/Homebrew/brew/blob/9ebcef785eed097be709bffef97263b5f536ba34/Library/Homebrew/cask/lib/hbc/cli/doctor.rb#L63-L66).

* The issue is reproducible in `brew irb`:

```
$ ls -dl ~/homebrew/Caskroom
drwxrwxr-x  10 luca  admin  340  9 Jun 16:22 /Users/luca/homebrew/Caskroom
$ ( cd ~ && pwd; )
/Users/luca
$ ls -dl /Users/luca/homebrew/Caskroom
drwxrwxr-x  10 luca  admin  340  9 Jun 16:22 /Users/luca/homebrew/Caskroom
$ brew irb
==> Interactive Homebrew Shell
Example commands available with: brew irb --examples
irb(main):001:0> Pathname.new("~/homebrew/Caskroom").exist?
=> false
irb(main):002:0> Pathname.new("/Users/luca/homebrew/Caskroom").exist?
=> true
```

mahori/brew,mgrimes/brew,MikeMcQuaid/brew,konqui/brew,JCount/brew,Linuxbrew/brew,ilovezfs/brew,EricFromCanada/brew,mgrimes/brew,vitorgalvao/brew,Homebrew/brew,konqui/brew,mahori/brew,JCount/brew,claui/brew,sjackman/homebrew,claui/brew,DomT4/brew,Homebrew/brew,MikeMcQuaid/brew,sjackman/homebrew,mgrimes/brew,ilovezfs/brew,JCount/brew,claui/brew,Linuxbrew/brew,MikeMcQuaid/brew,mahori/brew,Linuxbrew/brew,nandub/brew,maxim-belkin/brew,EricFromCanada/brew,sjackman/homebrew,DomT4/brew,reitermarkus/brew,nandub/brew,JCount/brew,MikeMcQuaid/brew,DomT4/brew,Homebrew/brew,vitorgalvao/brew,sjackman/homebrew,maxim-belkin/brew,vitorgalvao/brew,mahori/brew,nandub/brew,nandub/brew,konqui/brew,DomT4/brew,konqui/brew,EricFromCanada/brew,EricFromCanada/brew,reitermarkus/brew,reitermarkus/brew,ilovezfs/brew,reitermarkus/brew,maxim-belkin/brew,Linuxbrew/brew,Homebrew/brew,vitorgalvao/brew,claui/brew