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

@@ -17,7 +17,7 @@ const subTasks = {
     async dist(ctx) {
         const {dist, engines = {}} = await packageInfo('ghost', {
             version: ctx.version,
-            agent: getProxyAgent()
+            agent: {https: getProxyAgent()}
         });
 
         const skipNodeVersionCheck = (process.env.GHOST_NODE_VERSION_CHECK === 'false');