Update README logo URL

This now uses the canonical location for the logo image on the main Terraform website.

(The image isn't actually there at the time of this commit, but will be published here as
part of updating the website for the 0.10.0 release.)

README.md

@@ -5,7 +5,7 @@ Terraform
 - [![Gitter chat](https://badges.gitter.im/hashicorp-terraform/Lobby.png)](https://gitter.im/hashicorp-terraform/Lobby)
 - Mailing list: [Google Groups](http://groups.google.com/group/terraform-tool)
 
-![Terraform](https://rawgit.com/hashicorp/terraform-website/master/source/assets/images/logo-hashicorp.svg)
+![Terraform](https://www.terraform.io/assets/images/logo-hashicorp.svg)
 
 Terraform is a tool for building, changing, and versioning infrastructure safely and efficiently. Terraform can manage existing and popular service providers as well as custom in-house solutions.