document Travis CI automated tests configuration without Sauce Labs

[lemoustachiste]

Hi there,

I did struggle yesterday setting up automated testing on my PRs for my polymer project, so I thought adding the findings in the documentation would help more people finding the information quickly.

I am not going into too much details because I haven't experienced the need for it, but adding something about the custom wct.conf.json file could also be beneficial, for the reasons mentioned in this issue comment: https://github.com/Polymer/web-component-tester/issues/656#issuecomment-374068089

[googlebot]

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[lemoustachiste]

I signed it!

[googlebot]

CLAs look good, thanks!

[ghost]

Thanks! I will take a look as soon as possible

[ghost]

@aomarks Can I have a quick sanity-check on this from you if you have a chance? Just for technical info, not style etc

[aomarks]

Seems fine to me, but @azakus is more familiar with the details of how to get headless working.