fix(publish): exception when publishing 1st ver

When no versions are published, the lerna publish resulted the following exception:
```
lerna ERR! TypeError: Cannot read property '0.1.11' of undefined
lerna ERR!     at /home/circleci/project/node_modules/@lerna/publish/lib/get-unpublished-packages.js:20:31
lerna ERR!     at tryCatcher (/home/circleci/project/node_modules/bluebird/js/release/util.js:16:23)
lerna ERR!     at Promise._settlePromiseFromHandler (/home/circleci/project/node_modules/bluebird/js/release/promise.js:547:31)
```

commands/publish/lib/get-unpublished-packages.js

@@ -17,7 +17,7 @@ function getUnpublishedPackages(packageGraph, opts) {
   const mapper = pkg =>
     getPackument(pkg.name, opts).then(
       packument => {
-        if (packument.versions[pkg.version] === undefined) {
+        if (packument.versions && packument.versions[pkg.version] === undefined) {
           return pkg;
         }
       },