Adding automated update to docs on merges to master.

See https://travis-ci.org/dhermes/oauth2client/builds/44409213 for this in action.

@craigcitro You'll need to set some environment variables for the project
```
travis login --github-token=$(cat travis.token)
travis env set GH_OAUTH_TOKEN $(cat travis.token) --repo "google/oauth2client"
travis env set GH_OWNER google --repo "google/oauth2client"
travis env set GH_PROJECT_NAME oauth2client --repo "dhermes/oauth2client"
```

(More detailed instructions for Travis stuff on https://github.com/GoogleCloudPlatform/gcloud-python-wheels/blob/master/README.md if you're not familiar)