Use rootURL for default workerPath

[mnoble01]

Problem The default workerPath incorrectly points to the top-level file path. For any rootURL the workerPath is always:

https://example.com/assets/ace

Solution Because Ember assets are deployed relative to the rootURL, this addon should use the app's configuration when constructing the default workerPath. For example, given rootURL: "/my/awesome/app/", the workerPath should be:

https://example.com/my/awesome/app/assets/ace

Assumptions It is my understanding that app rootURLs always need a leading and trailing slash. If this is not the case, I'll have to update the solution to ensure a standard format.

[mnoble01]

@dfreeman I erased the git history since it didn't work, but I tried removing the --disable-gpu flag and adding useYarn: true and upgrading ember-try. There was one time the build succeeded after reverting some combination of those commits, but I couldn't reproduce the successful build again.