Update pandas.read_gbq docs to point to pandas-gbq
The `pandas-gbq` package must be installed to use `pandas.read_gbq`.
Also, with soon-to-be-release version 0.3.0 of `pandas-gbq` the Google
Cloud client library is used instead of the Google API library.

Also, standard SQL is no longer beta. In fact it is highly recommended
over using legacy SQL.

- `N/A` - closes #xxxx
- `N/A` (docs change only) - tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- `N/A` (docs change only) - whatsnew entry
