Build: Set "root: true" in eslintrc

Otherwise, the next version of qunitjs/jquery-release (which uses
ESLint instead of JSHint for linting its own code), is unable to
get "grunt" to pass on qunitjs/qunit because it's clone will be
in a subdirectory of itself, which then fails as follows:

> ~/Dev/qunit-release/__release/repo/test/reporter-html/window-onerror-preexisting-handler.html
>  9:27  error  'onerrorCallingContext' is assigned a value but never used  no-unused-vars

This is because ESLint is walking up from __release/repo and finding
the unrelated .eslintrc file in *that* repo.

.eslintrc.json

@@ -1,4 +1,5 @@
 {
+    "root": true,
     "env": {
         "es6": true
     },