Introducing custom pip install in tox to be used with cached wheels.

[dhermes]

In addition:

• Removes used of "pip install" in Travis config with our custom
  pip install.
• Introducing tox environment "coveralls" to encourage all Python
  dependencies to be contained in "tox.ini".
• Puts LICENSE header back on update_docs.sh and cleaned up some
  bash long line / syntax with shellchecker.

See GoogleCloudPlatform/gcloud-python-wheels#1 (comment)
step 2 for context.

---

This is towards a fix of #290 and the comment above describes the full path.

@silvolu After this is merged, I will restart the failed wheelhouse build
https://travis-ci.org/GoogleCloudPlatform/gcloud-python-wheels/builds/39208014
and the error

gcloud-python does not contain a custom pip install script.

will be resolved.

[dhermes]

Also I forgot to mention that this puts update_docs.sh in script instead of after_success.

This was requested by @silvolu in a private conversation and is the approach taken in gcloud-node.

[coveralls]

Coverage remained the same when pulling b01c883 on dhermes:speed-up-build-stage-one into d094d30 on GoogleCloudPlatform:master.

[dhermes]

For some more context, full integration with pre-built wheels can be finished via:
dhermes@3ae00ab

Commit 3ae00ab assumes that:

1. This commit has gone through (to define custom_pip_install.sh)
2. The build https://travis-ci.org/GoogleCloudPlatform/gcloud-python-wheels/builds/39208014 is re-run and a successfully built wheelhouse is committed to gcloud-python-wheels

[coveralls]

Coverage decreased (-100.0%) when pulling 8d1514d on dhermes:speed-up-build-stage-one into e1fd94f on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same when pulling 8c9aaaa on dhermes:speed-up-build-stage-one into e1fd94f on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same when pulling 5c346cf on dhermes:speed-up-build-stage-one into e1fd94f on GoogleCloudPlatform:master.

[silvolu]

Re-running the build.

[silvolu]

@dhermes the build has failed on try_update_wheelhouse.sh

[dhermes]

Oh shucks, I had never run it before with set -ev in push_wheels.sh and that failed because git add gcloud-0.02.2.whl exits with code 128 since *gcloud* is in .gitignore.

I think the best path forward to remove set -ev from push_wheels.sh. The other options that comes to mind is to implement a check of filenames against .gitignore when adding. This latter seems to be not good.

I'm looking into whether there is a simple way to get git add to exit with code 0 when encountering gitignored files and will update if I find something compelling.

[dhermes]

OK @silvolu I addressed this in GoogleCloudPlatform/gcloud-python-wheels#2

We can leave the failing build as failed after (hopefully) the new build passes. Then we'll move onto step two, which I've submitted as #308