.travis.yml: specify git -q

We are not interested in git(1) output. git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65721 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
2018-11-14 04:50:59 +00:00 · 2018-11-14 04:50:59 +00:00 · c2ef40d26a
--- a/.travis.yml
+++ b/.travis.yml
@ -24,6 +24,9 @@ osx_image: xcode10.1

 sudo: false

+git:
+  quiet: true
+
 addons:
  apt:
    config: