Default branches other than master ar not supported

[tomalec]

Do you want to request a feature or report a bug?

Bug

What is the current behavior?
When I run bundlewatch on Travis-CI for a push event, ci-env sets base branch as empty, then bundlewatch uses hardcoded default master, which does not exist in my repo

see https://app.travis-ci.com/github/woocommerce/google-listings-and-ads/jobs/522666112#L14735

[ERROR] Unable to fetch fileDetails for baseBranch=master from https://service.bundlewatch.io/store code=Request failed with status code 404
[INFO] master !== trunk, no results saved
[INFO] Saving results

I explicitly specified the tracked branches in my repo:

 "ci": {
            "trackBranches": ["trunk", "develop"]
        }

If the current behavior is a bug, please provide the steps to reproduce.

1. Setup bundlewatch for your repo, set custom trackBranches:

	"bundlewatch": {
		"files": [
			{
				"path": "./js/build/*.js",
				"maxSize": "3 kB"
			}
		],
        "ci": {
            "trackBranches": ["trunk", "develop"]
        }
	}

2. Run on travis script: npm run build && npx bundlewatch

(See our repo's bundlewatch config https://github.com/woocommerce/google-listings-and-ads/blob/trunk/package.json#L94-L120 and the build results https://app.travis-ci.com/github/woocommerce/google-listings-and-ads/jobs/522666112#L14735)

What is the expected behavior?

For a push even, when no CI_BASE_BRANCH is specified, should compare against CI_TARGET_BRANCH or at least respect the branch setup I gave, and use config.ci.trackBranches[0]

If this is a feature request, what is motivation or use case for changing the behavior?

n/a

Please mention other relevant information such as the browser version, Node.js version, bundlewatch version, and Operating System.