Limit Travis git checkout to last 5 commits

This is a forward port of PR #15016. Check it for details on why exactly commits.

Author: mzeis

.travis.yml

@@ -18,6 +18,8 @@ language: php
 php:
   - 7.1
   - 7.2
+git:
+  depth: 5
 env:
   global:
     - COMPOSER_BIN_DIR=~/bin