Exclude cloudprober from distribution

Hi

I noticed that this repository is installed when using `google/cloud-translate` packages and other `google/*` packages. It's listed as a dependency of `google/gax`

What is cloudprober? Does it need to be present in a composer distribution?

It looks to me like the following files can excluded via `.gitattributes`:
* `third_party/googleapis/`
* `cloudprober/`
* `doc/`

I can open a PR if this is OK.