travis: don't need git cloning progress

Since Travis runs as a batch job and logs are typically looked at
after-the-fact (if at all), there's not any point to seeing progress
when cloning git repos.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

.travis.yml

@@ -1,5 +1,7 @@
 git:
   depth: 10
+  quiet: true
+
 language: c
 dist: trusty
 sudo: false