[travis] Ensure that most recent tag is always available

Remove clone depth limit, to ensure that the most recent tag (from
which the version should be constructed) is always present.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

.travis.yml

 
 sudo: false
 
+git:
+  depth: false
+
 language: c
 
 cache: ccache