Add --HEAD support to Homebrew formula

This change adds support for --HEAD builds to the Homebrew formula. It
defines a conditional dependency on Maven and builds the -cli project
during install.  The location to install from is abstracted out so
that code is identical regardless of whether the build is --HEAD or
mainline.

spring-boot-cli/src/main/homebrew/springboot.rb

@@ -5,11 +5,23 @@ class Springboot < Formula
 	url 'https://repo.spring.io/${repo}/org/springframework/boot/spring-boot-cli/${project.version}/spring-boot-cli-${project.version}-bin.tar.gz'
 	version '${project.version}'
 	sha1 '${checksum}'
+	head 'https://github.com/spring-projects/spring-boot.git'
+
+	if build.head?
+		depends_on 'maven' => :build
+	end
 
 	def install
-		bin.install 'bin/spring'
-		lib.install 'lib/spring-boot-cli-${project.version}.jar'
-		bash_completion.install 'shell-completion/bash/spring'
-		zsh_completion.install 'shell-completion/zsh/_spring'
+		if build.head?
+			Dir.chdir('spring-boot-cli') { system 'mvn -U -DskipTests=true package' }
+			root = 'spring-boot-cli/target/spring-boot-cli-*-bin/spring-*'
+		else
+			root = '.'
+		end
+
+		bin.install Dir["#{root}/bin/spring"]
+		lib.install Dir["#{root}/lib/spring-boot-cli-*.jar"]
+		bash_completion.install Dir["#{root}/shell-completion/bash/spring"]
+		zsh_completion.install Dir["#{root}/shell-completion/zsh/_spring"]
 	end
 end