travis: Add a basic integration test.

[rillian]

This defines build instructions for the travis-ci.org continuous integration service which can provide feedback on commits and pull requests on github.

• Build on node v10.

  This is the current Long Term Support release so it's a good basis for testing.

  I tried to add current stable (v12.3.1) as well to get some advance notice of breakage, but it fails trying to compile the native fibres module.

• Explicitly invoke yarn and yarn build

  By default, the travis environment calls yarn test which isn't hooked up currently.

  The repo has yarn test:unit instead, but this fails with no tests configured.

• Cache yarn state for quicker results.

To make use of this file, code must be hosted on github and the specific repository added to travis. To do this, visit travis-ci.org, log in with github credentials, and authorize travis to access your github repositories.

Synchonize your account and visit the config for the specific repository under https://travis-ci.org/account/repositories to enable travis jobs for that project.

After that travis will run the build commands specified in this file on every new commit and post feedback on pull requests. This helps assure that code changes haven't broken something fundamental to the application.

[rillian]

Perhaps the netlify integration makes this redundant, but I find it useful because I can get test feedback on my own forks easily.

[rillian]

Ping on this, @paltman, @jtauber. Are you interested in contributions like this?

I'm develop a scaife viewer to integrate with the cdli. I know you're in the middle of a re-write, but I'd still prefer to work upstream if possible, especially since my time window is limited. It would be helpful to have some feedback on whether I'm heading in a useful direction.