update devDependencies

N.B.:
* `happiness` left heavily outdated
* though current svg2png has cli support we still need some script
  because buildin cli lacks overwrite option (domenic/svg2png#100)
  In our current setup `dist\images\toggle.png` has to be overwrited during build process

buildscripts/svgtopng.js

@@ -1,7 +1,7 @@
-var svg2png = require('svg2png');
-var ncp = require('ncp');
-var path = require('path');
+const fs = require("pn/fs"); // https://www.npmjs.com/package/pn
+const svg2png = require("svg2png");
 
-svg2png(path.resolve('src/images/toggle.svg'), path.resolve('src/images/toggle.png'), 1.0, function (err) {
-	ncp('src/images/toggle.png', 'dist/images/toggle.png');
-});
+fs.readFile("src/images/toggle.svg")
+    .then(svg2png)
+    .then(buffer => fs.writeFile("dist/images/toggle.png", buffer))
+    .catch(e => console.error(e));

package.json

@@ -21,13 +21,12 @@
     "url": "https://github.com/Norkart/Leaflet-MiniMap/issues"
   },
   "devDependencies": {
-    "clean-css": "3.0.7",
+    "clean-css-cli": "^4.2.1",
     "happiness": "^1.0.7",
-    "leaflet": ">=1.0.3",
-    "ncp": "1.0.1",
-    "svg2png": "1.1.0",
-    "svgo": "0.5.0",
-    "uglify-js": "2.6.0"
+    "leaflet": "^1.4",
+    "svg2png": "^4.1.1",
+    "svgo": "^1.2.0",
+    "uglify-js": "^3.4.10"
   },
   "scripts": {
     "build:js": "uglifyjs  --output dist/Control.MiniMap.min.js src/Control.MiniMap.js",