travis: only fetch top commit

This will make travis only fetch the top level commit instead of
fetching the top 50 (default). Nox should be able fetch other commits on
an as-needed basis.

.travis.yml

@@ -18,3 +18,5 @@ matrix:
           osx_image: xcode7.3
           script: ./maintainers/scripts/travis-nox-review-pr.sh pr
 install: ./maintainers/scripts/travis-nox-review-pr.sh install
+git:
+    depth: 1