Travis: if "make update" created a diff, please show it

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4652)

.travis.yml

@@ -168,7 +168,7 @@ script:
       else
           echo -e '+\057 MAKE UPDATE FAILED'; false;
       fi;
-      git diff --quiet
+      git diff --exit-code
     - if [ -n "$CHECKDOCS" ]; then
           if $make doc-nits; then
               echo -e '+\057\057 MAKE DOC-NITS OK';