Update deepmerge

Thanks TehShrike/deepmerge#84 :-).

package.json

@@ -25,9 +25,10 @@
     "addons-linter": "~0.28",
     "archiver": "~2.1",
     "chalk": "~2.3",
-    "deepmerge": "~1.5",
+    "deepmerge": "~2.0",
     "eslint": "~4.10",
     "fs-extra": "~4.0",
+    "is-mergeable-object": "~1.1",
     "jsdom": "~11.3",
     "one-svg-to-many-sized-pngs": "github:matatk/one-svg-to-many-sized-pngs",
     "rimraf": "~2.6",

scripts/build.js

@@ -2,10 +2,14 @@
 const path = require('path')
 const fse = require('fs-extra')
 const chalk = require('chalk')
-const deepmerge = require('deepmerge')
+const merge = require('deepmerge')
 const archiver = require('archiver')
 const oneSvgToManySizedPngs = require('one-svg-to-many-sized-pngs')
 const packageJson = require(path.join('..', 'package.json'))
+// For restoring pre-2.0.0 deepmerge behaviour...
+const isMergeableObject = require('is-mergeable-object')
+const emptyTarget = value => Array.isArray(value) ? [] : {}
+const clone = (value, options) => merge(emptyTarget(value), value, options)
 
 const extName = packageJson.name
 const extVersion = packageJson.version
@@ -125,9 +129,26 @@ function mergeManifest(browser) {
 	const commonJson = require(common)
 	const extraJson = require(extra)
 
+	function oldArrayMerge(target, source, optionsArgument) {
+		const destination = target.slice()
+
+		source.forEach(function(e, i) {
+			if (typeof destination[i] === 'undefined') {
+				const cloneRequested = !optionsArgument || optionsArgument.clone !== false
+				const shouldClone = cloneRequested && isMergeableObject(e)
+				destination[i] = shouldClone ? clone(e, optionsArgument) : e
+			} else if (isMergeableObject(e)) {
+				destination[i] = merge(target[i], e, optionsArgument)
+			} else if (target.indexOf(e) === -1) {
+				destination.push(e)
+			}
+		})
+		return destination
+	}
+
 	// Merging this way 'round just happens to make it so that, when merging
 	// the arrays of scripts to include, the compatibility one comes first.
-	const merged = deepmerge(extraJson, commonJson)
+	const merged = merge(extraJson, commonJson, { arrayMerge: oldArrayMerge })
 	merged.version = extVersion
 	fse.writeFileSync(
 		path.join(pathToBuild(browser), 'manifest.json'),