do not automatically download lfs files when cloning/checking out (matomo-org#13449)

* do not automatically download lfs files when cloning/checking out

* Tweak screenshots readme.

* Add lfs pull command.

* Update submodule.

* Regenerate .travis.yml.

* Must use --exclude= in fetch/pull command.

* Update submodule to merged master

Author: diosmosis

.lfsconfig

@@ -0,0 +1,2 @@
+[lfs]
+	fetchexclude = *

tests/README.screenshots.md

@@ -45,6 +45,10 @@ _Note: the `tests:sync-ui-screenshots` console command can be used to speed up t
 
 ## <a name="run-tests"></a>Running Tests
 
+Before running tests you'll want to pull the screenshots in lfs (the `--exclude=` since by default LFS files are excluded from being fetched in .lfsconfig to keep bandwidth usage down):
+
+    $ git lfs pull --exclude=
+
 You can test the UI by running the following command in the root piwik directory:
 
     $ ./console tests:run-ui