Revert "travis: fix clone depth"

This commit was supposed to fix travis CI build failures, caused by the
fact that abuild didn't ignore the exit status of git-describe(1), but
didn't. A proper fix for this issue has been committed in
1b18665c.

This reverts commit cb3f79af.

.travis.yml

 dist: trusty
 sudo: required
-git:
-  depth: false
 install:
   - sudo .travis/install-alpine
   - sudo .travis/setup-alpine