fixes to pkg uninstaller to prevent madness

 - do not remove *all* symlinks from referenced dirs, only broken ones
 - restore dir permission after use so we don't leave behind 777 dirs
 - add some better testing around `Cask::Pkg`
 - clean up `FakeSystemCommand` interface in tests

refs #1274
Merge pull request #1279 from phinze/1274-fix-pkg-uninstaller

fixes to pkg uninstaller to prevent madness
cut v0.19.3
Fix LittleSnitch Checksum

Closes #1283
Updated GPGMail 2.1
Merge pull request #1284 from desimone/patch-1

Updated GPGMail 2.1
Dolphin
Merge pull request #1285 from jonathantneal/Dolphin

Dolphin
Update Robomongo to 0.8.3
Merge pull request #1286 from julionc/robomongo-0.8.3

Update Robomongo to 0.8.3
Merge remote-tracking branch 'phinze/homebrew-cask/master'