Move `buildCount` to builder instance.
Move `buildCount` to builder instance.

When run normally (via `ember b` / `ember s`) there is only
a single `lib/models/builder.js` instance present, and therefore
the module scope integer is essentially the same as a per-instance
counter.

However, when running multiple builds within the same process (i.e.
our own test suite) this change allows the output path of the build data
to be much more consistent.

mike-north/ember-cli,trentmwillis/ember-cli,thoov/ember-cli,twokul/ember-cli,kanongil/ember-cli,asakusuma/ember-cli,jrjohnson/ember-cli,akatov/ember-cli,pzuraq/ember-cli,Turbo87/ember-cli,thoov/ember-cli,twokul/ember-cli,pzuraq/ember-cli,mike-north/ember-cli,jgwhite/ember-cli,balinterdi/ember-cli,romulomachado/ember-cli,fpauser/ember-cli,raycohen/ember-cli,calderas/ember-cli,fpauser/ember-cli,rtablada/ember-cli,scalus/ember-cli,scalus/ember-cli,calderas/ember-cli,cibernox/ember-cli,sivakumar-kailasam/ember-cli,jgwhite/ember-cli,akatov/ember-cli,asakusuma/ember-cli,seawatts/ember-cli,kriswill/ember-cli,Turbo87/ember-cli,scalus/ember-cli,seawatts/ember-cli,kriswill/ember-cli,patocallaghan/ember-cli,fpauser/ember-cli,scalus/ember-cli,ef4/ember-cli,twokul/ember-cli,gfvcastro/ember-cli,buschtoens/ember-cli,Turbo87/ember-cli,sivakumar-kailasam/ember-cli,HeroicEric/ember-cli,jgwhite/ember-cli,seawatts/ember-cli,kategengler/ember-cli,kanongil/ember-cli,cibernox/ember-cli,gfvcastro/ember-cli,twokul/ember-cli,sivakumar-kailasam/ember-cli,sivakumar-kailasam/ember-cli,pzuraq/ember-cli,kanongil/ember-cli,ember-cli/ember-cli,kellyselden/ember-cli,buschtoens/ember-cli,akatov/ember-cli,HeroicEric/ember-cli,mike-north/ember-cli,seawatts/ember-cli,kellyselden/ember-cli,akatov/ember-cli,romulomachado/ember-cli,rtablada/ember-cli,trentmwillis/ember-cli,kriswill/ember-cli,raycohen/ember-cli,patocallaghan/ember-cli,rtablada/ember-cli,HeroicEric/ember-cli,mike-north/ember-cli,kriswill/ember-cli,elwayman02/ember-cli,ef4/ember-cli,calderas/ember-cli,patocallaghan/ember-cli,gfvcastro/ember-cli,pzuraq/ember-cli,Turbo87/ember-cli,cibernox/ember-cli,trentmwillis/ember-cli,gfvcastro/ember-cli,ember-cli/ember-cli,rtablada/ember-cli,kellyselden/ember-cli,HeroicEric/ember-cli,ember-cli/ember-cli,kategengler/ember-cli,ef4/ember-cli,thoov/ember-cli,jrjohnson/ember-cli,patocallaghan/ember-cli,kellyselden/ember-cli,fpauser/ember-cli,thoov/ember-cli,calderas/ember-cli,balinterdi/ember-cli,elwayman02/ember-cli,kanongil/ember-cli,romulomachado/ember-cli,cibernox/ember-cli,jgwhite/ember-cli,ef4/ember-cli,romulomachado/ember-cli,trentmwillis/ember-cli