Grunt includes files outside src/ in build

[ryon]

If you run Grunt on this repo with a build/ directory already in place, it will start including files from outside of src/ in the build, including the previous build itself.

To reproduce:

1. Run grunt on a clean repo to produce a build directory. Note file size of slate-icons.svg.
2. Run grunt again.

Expected:
slate-icons.svg will be identical, as nothing has changed in src/

Actual:
slate-icons.svg has grown to include a copy of itself