Run unit tests for iOS code on CI server

[dersim-davaod]

NOTE: branch has been branched off from the 156-setup-travis-ci-to-have-a-build-jobs branch. It's more convenient to see the diff.

That means we'd prefer to merge #240 first and then I will change the base branch.

Here's the test results: https://travis-ci.com/github/mapbox/mapbox-vision-ios/builds/159633808

Checks:

• [ ] Update changelog
• [x] Rebase to dev branch
• [ ] Assign reviewers

Linked issues: Resolves #109

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.