Add --depth flag to 'git submodule update'

Fixes https://github.com/travis-ci/travis-ci/issues/1867
Fun fact:

``` Ruby
>> defined?(foo)
=> nil
>> if false
>>   foo = "bar"
>> end
>> defined?(foo)
=> "local-variable"
>> foo
=> nil
```

I think the ternary form might read a bit better, though.