Add xlsx-validator to travis build matrix

[vindvaki]

As promised in #501 (comment)

This adds a run of xlsx-validator on the files generated by examples/example.rb to the travis build matrix. You can see it in action on travis for my fork

These same steps can be used to run the validator locally as long as you have docker installed.

[vindvaki]

Oh and I guess your travis also uses the config from this PR for the build

[vindvaki]

Turns out the validator does not catch any of the corruption errors of current master, which is a bummer. So the added value seems pretty marginal.

[vindvaki]

Actually, it catches <a:round/>, paperHeight, and paperWidth, all of which caused trouble with Excel 2007. But it's also quite noisy:

• here are some issues that don't cause a corrupt document prompt in excel
• the main:sz error might be a false positive
• if the examples were split into smaller files it would be easier to pinpoint issues with individual features

[vindvaki]

@randym Could you please take a look? I've allowed failures for the validator build step, like you mentioned in #501.

[noniq]

@vindvaki This looks like a promising feature. If you’re still interested in getting this merged, please reopen this PR in https://github.com/caxlsx/caxlsx