Replace gulp.js with npm scripts

"gulp.js" (1) is meant to be used as streaming build tool for larger
projects with many tasks to combine them to a simple workflow. Nord Hyper
is a small project with only one build /development task where gulp.js is
more overhead / too heavy weight.

The "lint" task should be replaced with a simple npm script command to run "eslint".

References:

  (1) https://gulpjs.com

GH-27

docs/development/testing.md

@@ -5,7 +5,7 @@
 Nord Hyper uses [ESLint][eslint] to analyse the source code. To show the current lint status run
 
 ```sh
-gulp lint
+npm run lint
 ```
 
 [installation]: ../getting-started/installation.md

package.json

@@ -31,14 +31,12 @@
   "scripts": {
     "docs:build": "npm run docs:clean && gitbook install ./docs && gitbook build ./docs ./build/docs",
     "docs:clean": "node_modules/.bin/del ./build/docs",
-    "docs:dev": "npm run docs:build && gitbook serve ./docs ./build/docs"
+    "docs:dev": "npm run docs:build && gitbook serve ./docs ./build/docs",
+    "lint": "eslint --color index.js"
   },
   "devDependencies": {
     "del-cli": "1.1.0",
     "eslint": "4.15.0",
-    "gitbook-cli": "2.3.2",
-    "gulp": "3.9.1",
-    "gulp-eslint": "4.0.1",
-    "gulp-help": "1.6.1"
+    "gitbook-cli": "2.3.2"
   }
 }