Migrate to Travis CI

[c24t]

We're seeing a deprecation notice on https://circleci.com/gh/census-instrumentation/opencensus-python, time to upgrade.

[c24t]

See https://circleci.com/docs/2.0/workflows/#git-tag-job-execution for notes on updating our build-on-tag workflow.

[c24t]

Consider moving to travis, like the other OC client libraries.

[reyang]

@lzchen

[bogdandrutu]

OpenTelemetry is also going to use Travis.

[lzchen]

Travis CI gives us a lot of functionality and freedom for configurations, especially related to running different versions of Python. To take advantage of these customizations, we could consider moving our test automation from nox to tox (https://tox.readthedocs.io/en/latest/). There's a plugin available that integrates tox with TravisCI (https://tox-travis.readthedocs.io/en/stable/)

Nox's ability to run specific versions of Python per function can be replicated using functionality available in TravisCI. Nox also lacks a lot of the configurations that would be useful (such as caching dependencies). There's also the case of nox not being widely used so there's is very little support or documentation on it.

[lzchen]

There's also version specific logic that exists which will fail the coverage aggregation check (because nox aggregates results from different run versions). A possibility could be lowering the coverage threshold for aggregated test results.

[reyang]

@lzchen, what's your thinking on running test against different versions of python on local dev box?

[lzchen]

@reyang Running tests on different versions locally would be done sequentially, which might take a lot of time, especially with the installation of dependencies. With Travis, new virtual envs would be boot up at the same time and run in parallel and we can configure them to share from the same dependency list.

[bu3alwa]

Is this still needed? If so can I give it a go?