Use mkdirp to create build directory

package.json

@@ -10,7 +10,7 @@
     "browser-test-remote-ie": "nightwatch --config nightwatch.js --env ie",
     "browser-test-remote-safari": "nightwatch --config nightwatch.js --env safari",
     "browser-test-remote-parallel": "nightwatch --config nightwatch.js --env safari,ie,default,chrome",
-    "build": "mkdir build; browserify src/index.js -g yo-yoify -o build/app.js -t [ babelify ]; babel --plugins=transform-es2015-template-literals build/app.js --out-file build/app.js",
+    "build": "mkdirp build; browserify src/index.js -g yo-yoify -o build/app.js -t [ babelify ]; babel --plugins=transform-es2015-template-literals build/app.js --out-file build/app.js",
     "csslint": "csslint --ignore=order-alphabetical --errors='errors,duplicate-properties,empty-rules' --exclude-list='assets/css/font-awesome.min.css' assets/css/",
     "downloadsolc": "rm soljson.js; wget https://ethereum.github.io/solc-bin/soljson.js",
     "lint": "standard",
@@ -55,6 +55,7 @@
     "http-server": "0.9.0",
     "jquery": "^2.2.0",
     "js-base64": "^2.1.9",
+    "mkdirp": "^0.5.1",
     "nightwatch": "^0.9.3",
     "solc": "https://github.com/ethereum/solc-js",
     "standard": "^8.5.0",