Address "debug" npm audit issue

[plroebuck]

┌───────────────┬────────────────────────── │ Low │ Regular Expression Denial of Service ├───────────────┼────────────────────────── │ Package │ debug ├───────────────┼────────────────────────── │ Patched in │ >= 2.6.9 < 3.0.0 || >= 3.1.0 ├───────────────┼────────────────────────── │ Dependency of │ assetgraph-builder [dev] ├───────────────┼────────────────────────── │ Path │ assetgraph-builder > express-processimage > gm-papandreou > debug
├───────────────┼────────────────────────── │ More info │ https://nodesecurity.io/advisories/534 └───────────────┴──────────────────────────

Could "package.json" use debug-3.2.6; believe that version still supports Node-4.