Revert "travis: only fetch top commit"

This reverts commit 3c0fdefd84.

We have to keep more history because travis build could be
triggered after new commit is made, meaning it won't be able
to checkout the repository.

.travis.yml

@@ -15,8 +15,6 @@ matrix:
         - os: osx
           osx_image: xcode7.3
           script: ./maintainers/scripts/travis-nox-review-pr.sh nox pr
-git:
-    depth: 1
 env:
     global:
         - GITHUB_TOKEN=5edaaf1017f691ed34e7f80878f8f5fbd071603f