Add --depth flag to 'git submodule update'

Fixes https://github.com/travis-ci/travis-ci/issues/1867
Ah, that's a little curiosity in how Ruby works:

``` Ruby
>> foo = "hey" if false
=> nil
>> foo
=> nil
```

So `depth_opt` is `nil` if the conditional is falsey. I can change it around, but this way seemed to be the most readable to me.