test: Remove travis checkout depth

Tests on branches of non-head commits are failing, because the depth of
1 doesn't allow checking them out.

Remove `depth` as was the case before fa44af5cd2,
so that Travis can determine the minimum depth to check out.

.travis.yml

@@ -8,8 +8,6 @@ cache:
   - depends/built
   - depends/sdk-sources
   - $HOME/.ccache
-git:
-  depth: 1
 env:
   global:
     - MAKEJOBS=-j3