ci: remove the depth flag

This caused troubles because git-validation checks the commits until the very
initial one, which was unavailable when the `--depth` flag was passed by Travis
CI.

Signed-off-by: Miquel Sabaté Solà <msabate@suse.com>

.travis.yml

@@ -1,6 +1,9 @@
 language: nix
 sudo: true
 
+git:
+  depth: false
+
 env:
   # The minimum required plus the latest from each major release plus the next
   # release.