Enable travis and coveralls

[tmatsuo]

After merging #6

[tmatsuo]

weird, the build for the PR succeeded
https://travis-ci.org/GoogleCloudPlatform/python-docs-samples/builds/61875739

but the build for merge failed
https://travis-ci.org/GoogleCloudPlatform/python-docs-samples/builds/62574554

Those should have the same results.

[tmatsuo]

suspecting it's because tox 2.0 started to filter env variables
https://testrun.org/tox/latest/example/basic.html#passing-down-environment-variables

[tmatsuo]

Fixed by #15