re-enable heimdall build

https://github.com/ember-cli/loader.js/blob/master/index.js#L14

## Summary

I disable the heimdall stuff, because I believe I was seeing issues with it locally. Those issues were related to it appearing like the heimdall was *sometimes* being loaded when I did not enable it. It may have been an unrelated problem, and needs reinvestigation.

## Objective

Restore the heimdal build + env flag

Start at index.js line 14 and inspect how the Heimdall build and environment flag were disabled. Reinvestigate whether Heimdall is loaded without being enabled, then restore the build and flag; completion means the build is available and the unintended-loading behavior is understood or resolved.

Written by the indexing model from the issue text.