[WIP] Include Danger in travis-ci

[lgarithm]

Checklist

• I've tested that my changes are compatible with the latest version of Tensorflow.
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

#562

[lgarithm]

Let's see how it works for a PR.

[lgarithm]

Looks like it's working.

And according to our settings, failed to pass danger check won't block merge.

[lgarithm]

The remaining thing TODO is just introduce the CHANGELOG.md file,
and rewrite Dangerfile appropriately.

[lgarithm]

@luomai the danger check provides a way of running additional checks within travis-ci.
We were searching for such a solution (a way to add warning message in test, but not fail to test.) when fixing pylints.

[DEKHTIARJonathan]

@lgarithm I have added a template for the Changelog.md

I will update it with the relevant information for the 1.8.5 release

[lgarithm]

	1 Error
🚫	Please include a CHANGELOG entry. You can find it at CHANGELOG.md.

	1 Message
📖	Note, we hard-wrap at 80 chars and use 2 spaces after the last line.

Generated by 🚫 Danger

[DEKHTIARJonathan]

@zsdonghao @lgarithm we should try a merge. It seems that we need to have the Changelog inside the repository for it to work.

Is it fine if I merge and we'll see if it works ? We can still rollback if it doesn't.

I have modified in the PR :

• Danger will only run with Python 3.6 on pull requests.
• I have added a template Changelog
• Added a few custom configs to danger i found in various places.

[DEKHTIARJonathan]

After a talk with @zsdonghao I merge this PR

[2wins]

@DEKHTIARJonathan This PR seems to have a problem. Any PR cannot pass the Travis-CI test now.

[DEKHTIARJonathan]

Problem with API key I'll fix it tonight. I don't have my computer now