Make sure the build traps `SIGINT` and `SIGTERM` for cleanup.

Exiting from ```ember server``` stopped cleaning up the ```./tmp``` directory after exit, which means the directory would expand in size until manually deleted.

***MIGHT*** have caused some EMFILE errors as well.  I have no proof. Though.

Traces back to a mis-refactor from https://github.com/stefanpenner/ember-cli/blob/v0.0.28/lib/utilities/build-builder.js#L16