Added devDependency on 'yarn' to enable production build.

package.json

@@ -76,7 +76,8 @@
     "sinon": "^7.2.3",
     "sinon-chai": "^3.3.0",
     "targz": "^1.0.1",
-    "temp": "^0.9.0"
+    "temp": "^0.9.0",
+    "yarn": "^1.15.2"
   },
   "config": {
     "platformDependentModules": {

yarn.lock

@@ -5446,6 +5446,11 @@ yargs@^7.1.0:
     y18n "^3.2.1"
     yargs-parser "^5.0.0"
 
+yarn@^1.15.2:
+  version "1.15.2"
+  resolved "https://registry.yarnpkg.com/yarn/-/yarn-1.15.2.tgz#7a064ca81ca34235f16376ad2f796ed432f9e285"
+  integrity sha512-DhqaGe2FcYKduO42d2hByXk7y8k2k42H3uzYdWBMTvcNcgWKx7xCkJWsVAQikXvaEQN2GyJNrz8CboqUmaBRrw==
+
 yauzl@2.4.1:
   version "2.4.1"
   resolved "https://registry.yarnpkg.com/yauzl/-/yauzl-2.4.1.tgz#9528f442dab1b2284e58b4379bb194e22e0c4005"