DOC: Updated sample CI image in contributing docs
The image of a green CI in https://pandas-docs.github.io/pandas-docs-travis/contributing.html#testing-with-continuous-integration became outdated (contains circleci and appveyor). Replaced by a current one with only travis and azure.

![pandas_ci_doc_screenshot](https://user-images.githubusercontent.com/10058240/50937863-8d87f880-146d-11e9-8a3b-6c32bcd25e36.png)

