Add installation of grakn.ai

Add suggestion from MikeMcQuaid

Formula/grakn.rb

@@ -0,0 +1,18 @@
+class Grakn < Formula
+  desc "The Database for AI"
+  homepage "https://grakn.ai"
+  url "https://github.com/graknlabs/grakn/releases/download/v0.12.1/grakn-dist-0.12.1.tar.gz"
+  sha256 "3658138ccd74e51a0072a42f5e4fff43763506b02e581b572b05f50b1c63eb72"
+
+  depends_on :java => "1.8+"
+
+  def install
+    libexec.install Dir["*"]
+    bin.install Dir["#{libexec}/bin/*"]
+    bin.env_script_all_files(libexec/"bin", :CASSANDRA_HOME => ENV["CASSANDRA_HOME"])
+  end
+
+  test do
+    assert_match /stopped/i, shell_output("#{bin}/grakn.sh status")
+  end
+end