brew audit fails to install rubocop

brew config.out

HOMEBREW_VERSION: 1.8.4-25-gc89f6c8
ORIGIN: https://github.com/Homebrew/brew
HEAD: c89f6c8f8c60c52a9278ccb035dadf73d999df13
Last commit: 4 hours ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: 62dda3a6286a6f0ef7378e085cd0b7c202b41db1
Core tap last commit: 2 hours ago
HOMEBREW_PREFIX: /usr/local
HOMEBREW_DEV_CMD_RUN: 1
HOMEBREW_VISUAL: subl
CPU: quad-core 64-bit haswell
Homebrew Ruby: 2.3.7 => /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby
Clang: 10.0 build 1000
Git: 2.17.2 => /Applications/Xcode.app/Contents/Developer/usr/bin/git
Curl: 7.54.0 => /usr/bin/curl
Java: 11
macOS: 10.14.1-x86_64
CLT: 10.1.0.0.1.1539992718
Xcode: 10.1
XQuartz: 2.7.11 => /opt/X11

brew doctor.out

Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: Homebrew/homebrew-core is not on the master branch

Check out the master branch by running:
  git -C "$(brew --repo homebrew/core)" checkout master

Warning: The filesystem on / appears to be case-sensitive.
The default macOS filesystem is case-insensitive. Please report any apparent problems.