Update README.md in preparation for cutting 0.9.1 (#129)

At some point should really find a better way, but whatever, we don't
cut releases that often.

README.md

diff --git a/README.md b/README.md
index a4d40af..063b31b 100644
--- a/README.md
+++ b/README.md
@@ -117,7 +117,7 @@
 2. Install keep-sorted:
 
    ```sh
-   $ go install github.com/google/keep-sorted@v0.9.0
+   $ go install github.com/google/keep-sorted@v0.9.1
    ```
 
 3. Run keep-sorted:
@@ -135,7 +135,7 @@
 
 ```yaml
 - repo: https://github.com/google/keep-sorted
-  rev: v0.9.0
+  rev: v0.9.1
   hooks:
     - id: keep-sorted
 ```