MDL-63346 travis: GRUNT job immune to npm-shrinkwrap.json changes

When upgrading nodejs/npm versions, there are modifications to
different packages or to the format of the npm-shrinkwrap.json
file. Usually that leads to failed jobs untill the bundled file
is updated.

To avoid that, we just ignore the npm-shrinkwrap.json file, so
just the REAL modifications in css/js/ignore stuff are considered.

.travis.yml

@@ -263,7 +263,9 @@ script:
         grunt ;
         # Add all files to the git index and then run diff --cached to see all changes.
         # This ensures that we get the status of all files, including new files.
+        # We ignore npm-shrinkwrap.json to make the tasks immune to npm changes.
         git add . ;
+        git reset -- npm-shrinkwrap.json ;
         git diff --cached --exit-code ;
       fi