Install gRPC et al for testing except py26/lint/docs-rtd

[tseaver]

Rationalize tox vs. gRPC:

• Install grpcio and related deps by default, and omit adding _testing to PYTHONPATH.
• For Python 2.6, and lint, RTD, suppress installing gRPC-related deps, and add _testing to PYTHONPATH.

Bump minimum version for grpcio to 0.15.0.

Fix up Py3k failures with grpcio installed.

@daspecster, @dhermes Maybe this should land before 0.17.0?

[tseaver]

I'm going to go ahead with 0.17.0 without this one.

[dhermes]

Changes look mostly good. Failures caused by what?

[tseaver]

Ugh, no idea -- I can't reproduce on my Ubuntu Trusty machine here. "Two hard things in computer science: caching, naming things, and off-by-one errors."

[daspecster]

@tseaver it did the same thing to me on OSX, someone reported it here and I opened an issue on grpc.
However it looks like it was two different things.

I believe grpc needs to do a release before this works again.
grpc/grpc#7135 (comment)

How I reproduce the issue.

I made a virtualenv with py34.

mkvirtualenv -p /usr/local/bin/python3.4 test-multi
python setup.py install
pip install tox
tox -e py34

Key points from the log output that it mentions in the error...

Command "/Users/me/Documents/google/gcloud-python/.tox/py34/bin/python3.4 -u -c "import setuptools, 
tokenize;__file__='/private/var/folders/bc/htqxjjns6gv4g1kspb2zppm80000gn/T/pip-build-
bifiqtz2/grpcio/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), 
__file__, 'exec'))" install --record /var/folders/bc/htqxjjns6gv4g1kspb2zppm80000gn/T/pip-ndumdegl-
record/install-record.txt --single-version-externally-managed --compile --install-headers 
/Users/me/Documents/google/gcloud-python/.tox/py34/bin/../include/site/python3.4/grpcio" failed with 
error code 1 in /private/var/folders/bc/htqxjjns6gv4g1kspb2zppm80000gn/T/pip-build-bifiqtz2/grpcio/

[daspecster]

@tseaver, should we just close this for now and hope that future grpc fixes it?

[tseaver]

@daspecster

Should we just close this for now and hope that future grpc fixes it?

I think we want to be where this PR is (the rationalize tox.ini, particularly). We need to have grpcio wheels published for Python 3.4+ (including Windows) before our story improves.

[tseaver]

@dhermes, @daspecster I rebased to fix conflicts, and also updated the version pins for the gRPC-related dependencies to reflect today's grpcio-1.0rc1 release, as well as comments from @bjwatson on potential future breaking changes for the GAX libraries.

[tseaver]

Woohoo! Travis is happy, at least for the PR.

[tseaver]

@dhermes, @daspecster any remaining issues?

[dhermes]

LGTM