travis-ci: set git fetch depth to fix git describe

It is invoked by packpack to set a version of a package.

Part of #49.

Author: Totktonada

.travis.yml

@@ -9,6 +9,9 @@ cache:
     directories:
       - $HOME/.cache
 
+git:
+    depth: 100500
+
 env:
     global:
       - PRODUCT=tarantool-memcached