Ignore last_modified timestamp deciding whether to do an update (#9)

The `last_modified` timestamp is tied to when a user did a `terraform
init`. This means the exact same code, checked out at different times,
results in a change being detected. Ignoring the time_stamp fixes this.

main.tf

@@ -53,6 +53,9 @@ resource "aws_lambda_function" "notify_slack" {
   }
 
   lifecycle {
-    ignore_changes = ["filename"]
+    ignore_changes = [
+      "filename",
+      "last_modified",
+    ]
   }
 }