fix(install): pass agent correctly to packageJson function

closes TryGhost#1899
- at some point the syntax changed, so we need to pass the agent differently

Author: acburdine

lib/tasks/yarn-install.js

@@ -15,9 +15,10 @@ const getProxyAgent = require('../utils/get-proxy-agent');
 
 const subTasks = {
     async dist(ctx) {
+        const proxyAgent = getProxyAgent();
         const {dist, engines = {}} = await packageInfo('ghost', {
             version: ctx.version,
-            agent: getProxyAgent()
+            agent: proxyAgent ? {https: proxyAgent} : false
         });
 
         const skipNodeVersionCheck = (process.env.GHOST_NODE_VERSION_CHECK === 'false');