closure-compiler: requires Java 7

Closes Homebrew/homebrew#27938.

Formula/closure-compiler.rb

@@ -1,5 +1,17 @@
 require 'formula'
 
+class Java7 < Requirement
+  fatal true
+
+  def satisfied?
+    `java -version`.split("\n")[0] =~ /"1\.7/
+  end
+
+  def message
+    "Java 7 is required."
+  end
+end
+
 class ClosureCompiler < Formula
   homepage 'http://code.google.com/p/closure-compiler/'
   url 'https://code.google.com/p/closure-compiler/', :using => :git, :tag => 'v20140303'
@@ -7,6 +19,7 @@ class ClosureCompiler < Formula
   head 'https://code.google.com/p/closure-compiler/', :using => :git
 
   depends_on :ant
+  depends_on Java7
 
   def install
     system "ant", "clean"