ISSUE-58: Remove usage of deprecated Client.dataset

[dimkonko]

Use TableReference instead.

Issue: #58

[tswast]

Thanks for the fix!

[tswast]

Hmmm. Not sure why the tests aren't running against the PR.

[tswast]

Tests pass locally:

$ gh pr checkout 59

$ pytest test
====================================== test session starts =======================================
platform darwin -- Python 3.6.10, pytest-3.2.2, py-1.9.0, pluggy-0.4.0
rootdir: /Users/swast/src/pybigquery, inifile: setup.cfg
collected 63 items

test/test_dialect_types.py .
test/test_parse_url.py ....................................
test/test_sqlalchemy_bigquery.py ...
.
.
.
=================================== 63 passed in 69.75 seconds ===================================

Thank you @dimkonko for the contribution!