Upgrading version to 0.4.2.

[dhermes]

This is for a minor release cycle.

[coveralls]

Coverage remained the same at 100.0% when pulling 91167a2 on dhermes:cut-0.4.2 into 0e41dc3 on GoogleCloudPlatform:master.

[dhermes]

Running

$ git log 0.4.1..91167a2bf8c1364e2942127f1a75a2b86bef6704 \
> | grep 'Merge pull request ' \
> | awk -F ' ' '{print $4}' \
> | sort

yields:

#641
#645
#655
#656
#660
#662
#663
#664
#665
#666
#667
#668
#670
#673
#674
#676
#677

[dhermes]

This is my plan for the release notes on the tag:

-   Adding get_for_service_account_json function to allow JSON
    service accounts without `GOOGLE_APPLICATION_CREDENTIALS`.
-   Adding [support][2] for GCD datastore [dev server][1].
-   Adding `Dataset` class to act as a factory containing a
    preset connection and dataset ID. (Existed several releases
    ago in a different form.)
-   Adding lazy loading support for default dataset ID and
    connection in `datastore` package. This allows auth to be done
    without needing to call `datastore.set_defaults()` provided
    the correct environment variables are set.

[1]: https://cloud.google.com/datastore/docs/tools/devserver
[2]: https://github.com/GoogleCloudPlatform/gcloud-python/pull/656

The only other changes are small bugfix (#677), test related (#676) and many for docs spread around.

[tseaver]

LGTM

[dhermes]

Canceled https://travis-ci.org/GoogleCloudPlatform/gcloud-python/builds/52167151 so it doesn't conflict with this docs build. (Was for #680). Due to current Travis outage: https://twitter.com/traviscistatus/status/570637064019914752

[dhermes]

Tag pushed, release created and waiting on Travis