Drop support for Python 2.6.

[tseaver]

Closes #1607.

[dhermes]

I'm grep-ing around for some other things worth cleaning up:

CONTRIBUTING.rst:- The feature must work fully on the following CPython versions: 2.6,
CONTRIBUTING.rst:-  `Python 2.6`_
CONTRIBUTING.rst:.. _Python 2.6: https://docs.python.org/2.6/
CONTRIBUTING.rst:We may `drop 2.6`_ as a supported version as well since Python 2.6 is no
CONTRIBUTING.rst:.. _drop 2.6: https://github.com/GoogleCloudPlatform/gcloud-python/issues/995
gcloud/_helpers.py:363:    if sys.version_info[:2] < (2, 7):  # pragma: NO COVER Python 2.6
gcloud/exceptions.py:40:        # suppress deprecation warning under 2.6.x
gcloud/error_reporting/client.py:174:            # once py26 support is dropped this can use dict comprehension

[tseaver]

@dhermes Should be addressed in subsequent commits.

[tseaver]

Amended to fix the lint issue.

[dhermes]

Only question left: do we want to add a warning on import?

[tseaver]

Do we want to add a warning on import?

I think not on import: maybe on installation. @daspecster, @jgeewax WDYT?

[dhermes]

OK let's just merge (LGTM) and file an issue to follow up / decide if we should warn on either import or installation.

[tseaver]

See also #2056.