Changing destDir breaks the `ember staticboot` command

[mike183]

It seems that setting a custom destDir in an applications ember-cli-build.js file causes the ember staticboot command to start returning 404 errors.

I've tracked the issue down to the staticboot-server task being hard coded to serve dist/staticboot here, rather than using the user configured destDir.

I did look at submitting a PR but was unsure of the best way to to retrieve the user configured options. If you could point me in the direction, I can take another look.