Merge pull request aws#663 from dstevenson/bugfix/guzzle-6-classname

Updated Guzzle5 classname to Guzzle6 classname for CachingStream

src/Exception/CouldNotCreateChecksumException.php

@@ -11,7 +11,7 @@ public function __construct($algorithm, \Exception $previous = null)
             . "seekable. To prevent this error you can either 1) include the "
             . "ContentMD5 or ContentSHA256 parameters with your request, 2) "
             . "use a seekable stream for the body, or 3) wrap the non-seekable "
-            . "stream in a GuzzleHttp\\Stream\\CachingStream object. You "
+            . "stream in a GuzzleHttp\\Psr7\\CachingStream object. You "
             . "should be careful though and remember that the CachingStream "
             . "utilizes PHP temp streams. This means that the stream will be "
             . "temporarily stored on the local disk.", 0, $previous);