Updating README to make EXTRA_TOX_ENVS public. [ci skip]

[dhermes]

@tseaver PTAL. This is to agree with googleapis/google-cloud-python#309

[dhermes]

I added ci skip since we don't want to waste Travis resources building a new wheelhouse.

[tseaver]

I don't know enough about the semantics of '--public' to verify here.

[dhermes]

$ travis env -h
Show or modify build environment variables.
Usage: travis env list [OPTIONS]
travis env set NAME VALUE [OPTIONS]
travis env unset [NAMES..] [OPTIONS]
travis env copy [NAMES..] [OPTIONS]
-h, --help Display help
-i, --[no-]interactive be interactive and colorful
-E, --[no-]explode don't rescue exceptions
--skip-version-check don't check if travis client is up to date
--skip-completion-check don't check if auto-completion is set up
-e, --api-endpoint URL Travis API server to talk to
-I, --[no-]insecure do not verify SSL certificate of API endpoint
--pro short-cut for --api-endpoint 'https://api.travis-ci.com/'
--org short-cut for --api-endpoint 'https://api.travis-ci.org/'
--staging talks to staging system
-t, --token [ACCESS_TOKEN] access token to use
--debug show API requests
-X, --enterprise [NAME] use enterprise setup (optionally takes name for multiple setups)
--adapter ADAPTER Faraday adapter to use for HTTP requests
-r, --repo SLUG repository to use (will try to detect from current git clone)
-R, --store-repo SLUG like --repo, but remembers value for current directory
-P, --[no-]public make new values public
-p, --[no-]private make new values private
-u, --[no-]unescape do not escape values

[dhermes]

@silvolu Can you give this a look?

[silvolu]

You want to make the value visible in the build report and the settings right? If that's the case, LGTM.

[dhermes]

Yeah. The logic being that this will change more often than the others and a maintainer may forget the value.

It depends on the tox environments not included in envlist that will be run by Travis.