Prefer ES5 methods over underscore utilities

Since 07cc454 (ie the switch to Meteor 1.2) we includes the `es5-shim`
polyfill to support methods like `Array.prototype.forEach` in a
consistent way across all supported browsers (IE8+).

MDG recently released a blog post recommending the use of these native
methods instead of underscore [0]. We know follow this recommendation.

This commit also favor some ES6 features (argument defaults,
destructing assignment) in places where we didn’t use them.

[0]: http://info.meteor.com/blog/es2015-get-started

models/boards.js

@@ -279,7 +279,7 @@ Boards.before.insert((userId, doc) => {
   // Handle labels
   const colors = Boards.simpleSchema()._schema['labels.$.color'].allowedValues;
   const defaultLabelsColors = _.clone(colors).splice(0, 6);
-  doc.labels = _.map(defaultLabelsColors, (color) => {
+  doc.labels = defaultLabelsColors.map((color) => {
     return {
       color,
       _id: Random.id(6),