cweb: add livecheck

[chenrui333]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

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add livecheck to improve the check result

$ brew livecheck cweb
- cweb : 4.5 ==> 2021-07-14
+ cweb : 4.5 ==> 4.5

[BrewTestBot]

🤖 A scheduled task has triggered a merge.