gradle: set GRADLE_HOME in wrapper script. (#1543)

Also, cleanup the test a little.

As mentioned in #889.

Formula/gradle.rb

@@ -12,14 +12,16 @@ class Gradle < Formula
 
   bottle :unneeded
 
+  depends_on :java => "1.6+"
+
   def install
     libexec.install %w[bin lib]
-    bin.install_symlink libexec+"bin/gradle"
+    (bin/"gradle").write_env_script libexec/"bin/gradle",
+      Language::Java.java_home_env.merge(:GRADLE_HOME => libexec)
   end
 
   test do
     ENV.java_cache
-    output = shell_output("#{bin}/gradle --version")
-    assert_match /Gradle #{version}/, output
+    assert_match(/Gradle #{version}/, shell_output("#{bin}/gradle --version"))
   end
 end