$ brew tap henrytseng/homebrew-elasticsearch
==> Tapping henrytseng/elasticsearch
Cloning into '/usr/local/Homebrew/Library/Taps/henrytseng/homebrew-elasticsearch'...
remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (5/5), done.
remote: Total 5 (delta 0), reused 5 (delta 0), pack-reused 0
Unpacking objects: 100% (5/5), done.
Tapped 1 formula (31 files, 33.7KB).

$ brew install elasticsearch15
==> Installing elasticsearch15 from henrytseng/elasticsearch
==> Downloading https://download.elastic.co/elasticsearch/elasticsearch/elasticsearch-1.5.2.tar.gz
######################################################################## 100.0%
Error: Failed to install plist file
Error: undefined method `undent' for #<String:0x00007f85d027d190>
Please report this bug:
  https://docs.brew.sh/Troubleshooting
/usr/local/Homebrew/Library/Taps/henrytseng/homebrew-elasticsearch/elasticsearch15.rb:72:in `caveats'
/usr/local/Homebrew/Library/Homebrew/caveats.rb:19:in `caveats'
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/2.3.0/forwardable.rb:202:in `empty?'
/usr/local/Homebrew/Library/Homebrew/formula_installer.rb:618:in `caveats'
/usr/local/Homebrew/Library/Homebrew/formula_installer.rb:667:in `finish'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:332:in `install_formula'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:263:in `block in install'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:261:in `each'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:261:in `install'
/usr/local/Homebrew/Library/Homebrew/brew.rb:102:in `<main>'

$ brew -v
Homebrew 2.1.10
Homebrew/homebrew-core (git revision 607b6; last commit 2019-08-17)
Homebrew/homebrew-cask (git revision 56b6c; last commit 2019-08-17)

$ ruby -v
ruby 2.3.7p456 (2018-03-28 revision 63024) [universal.x86_64-darwin18]