Travis PR commit sha is not correctly gathered for status checks with GitHub Integration

[jennifer-shehane]

Current behavior:

Cypress associates the PR for the GitHub integration by the commit SHA sent via environment variables from the CI provider. Travis's docs mentions sending these a bit different than other CI providers here:

Rather than build the commits that have been pushed to the branch the pull request is from, we build the merge between the source branch and the upstream branch.
https://docs.travis-ci.com/user/pull-requests/

Currently Cypress looks for TRAVIS_COMMIT, but supplying the TRAVIS_PULL_REQUEST_SHA (https://docs.travis-ci.com/user/environment-variables/) resolves this SHA mixup.

Desired behavior:

A status check for GitHub should run on both commit and PR initiated build from Travis.

Steps to reproduce: (app code and test code)

Some experimenting here: cypress-io/cypress-example-kitchensink#328

Versions

3.6.1

[cypress-bot]

The code for this is done in cypress-io/cypress#5714, but has yet to be released.
We'll update this issue and reference the changelog when it's released.

[cypress-bot]

Released in 3.7.0.