Update package to support Guzzle 6 and stop supporting old versions.

src/HttpSignatures/GuzzleHttpSignatures.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace HttpSignatures;
+
+use GuzzleHttp\HandlerStack;
+use GuzzleHttp\Psr7\Request;
+
+class GuzzleHttpSignatures
+{
+    /**
+     * @param Context $context
+     * @return HandlerStack
+     */
+    public static function defaultHandlerFromContext(Context $context)
+    {
+        $stack = HandlerStack::create();
+        $stack->push(self::middlewareFromContext($context));
+
+        return $stack;
+    }
+
+    /**
+     * @param Context $context
+     * @return \Closure
+     */
+    public static function middlewareFromContext(Context $context)
+    {
+        return function (callable $handler) use ($context)
+        {
+            return function (
+                Request $request,
+                array $options
+            ) use ($handler, $context)
+            {
+                $request = $context->signer()->sign($request);
+                return $handler($request, $options);
+            };
+        };
+    }
+}