Add vcddiff to Travis

[veripoolbot]

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Author Name: Todd Strader (@toddstrader)
Original Redmine Issue: 1600 from https://www.veripool.org

Original Assignee: Todd Strader (@toddstrader)

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A number of tests are not running because this is missing.

[veripoolbot]

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Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2019-11-15T07:59:22Z

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Might want to put a binary somewhere as this almost never changes.

[veripoolbot]

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Original Redmine Comment
Author Name: Todd Strader (@toddstrader)
Original Date: 2019-12-10T10:31:59Z

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Ideally I would have created a PPA for this (I found one, but it doesn't work for some reason). However, I've never done that before and the instructions were quite complicated. Instead, I found that adding a script that pulls, compiles and installs vcddiff takes 1 second in Travis. So it's unclear if we'd really even save any time by using apt instead.