due to the change of visjs url, change to another cdn of visjs

[daviddhc20120601]

fix issue 328 link:#328

due to the change of visjs url, change to another cdn of visjs

from

<script src="http://visjs.org/dist/vis.js"></script>
<link href="http://visjs.org/dist/vis-timeline-graph2d.min.css" rel="stylesheet" type="text/css" />

to

https://cdnjs.cloudflare.com/ajax/libs/vis/4.21.0/vis.js
https://cdnjs.cloudflare.com/ajax/libs/vis/4.21.0/vis.min.css

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[daviddhc20120601]

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[daviddhc20120601]

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@googlebot I signed it!

#352

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CLAs look good, thanks!

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[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

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• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
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[ryanmcdowell]

@googlebot I consent

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A Googler has manually verified that the CLAs look good.

(Googler, please make sure the reason for overriding the CLA status is clearly documented in these comments.)

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