package.json: remove `npm run coverage` from `npm test`.

There isn't any point in regenerating coverage reports after every test
run.

package.json

@@ -8,7 +8,7 @@
   "main": "index.js",
   "scripts": {
     "start": "node index.js",
-    "test": "npm run unit && npm run coverage",
+    "test": "npm run unit",
     "unit": "node test/unit/run.js | tap-spec",
     "ciao": "node node_modules/ciao/bin/ciao -c test/ciao.json test/ciao",
     "coverage": "node_modules/.bin/istanbul cover test/unit/run.js",