Improve integration readme

Maybe obvious, but not really: I had an older set of deps installed and my integration tests were failing because of that.

It took me 30 seconds to fix, but maybe this readme line will save that time to millions of developers, saving some trees as the end result! :)

Author: janklan

tests/Integration/README.md

@@ -27,6 +27,8 @@ docker run --name postgres-doctrine-test -e POSTGRES_PASSWORD=postgres -e POSTGR
 ### Run the Tests
 
 ```bash
+# Install the latest dependencies
+composer update
 # Run the integration tests
 composer run-integration-tests
 ```