.travis.yml: Detect if 'make update' updated something

If it did, it really is something that should be checked in, and should
therefore make a CI build fail.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3686)

.travis.yml

@@ -184,7 +184,8 @@ script:
     - if [ -n "$DESTDIR" ]; then
           cd _build;
       fi
-    - $make update
+    - $make update;
+      git diff --quiet
     - if [ -n "$CHECKDOCS" ]; then
           $make doc-nits;
       fi