Use version generated from parent version strategy if no version supplied

[jkeen]

This addon states in its config that versioning should be done using versionStrategy in the parent ember-service-worker addon.

// Changing this version number will bust the cache, but you probably do not
// want to be doing this manually, but rather using `versionStrategy` as
// explained here http://ember-service-worker.com/documentation/configuration/#versioning
version: '1',

Unfortunately removing the version flag from the config block as implied just defaults the version to 1 and does not change the local version or bust the cache, despite the parent version number changing due to the versionStrategy. This PR pulls in the parent version number if the local version is not set.

[chbonser]

@martndemus mind taking a look at this PR and seeing if it makes sense to merge? There is a similar PR for the asset cache as well DockYard/ember-service-worker-asset-cache#46.